A blood screening was a type of blood test that was initially popular as a means of determining if an individual was a Changeling. It was used through the logic that once blood was drawn from a subject, if they were a Changeling, their matter would revert to its gelatinous state.
A changeling is typically identifiable via a number of traits, which vary from culture to culture. In Irish legend, a fairy child may appear sickly and will not grow in size like a normal child, and may have notable physical characteristics such as a beard or long teeth.
Some changelings in German Folklore were seen to have large heads and small bodies. In the highlands, the changeling had large teeth, a large appetite, a love of music, and could dance and be known to be precocious. Changelings have an adult intellect and personality which they try to hide.
Most changelings don't know they're changelings.
Though they may always feel somewhat different or removed from human life, most changelings are too young to understand who they truly are.
The true form of a changeling is a bit paler than a typical human with slightly elongated limbs. Their eyes are colorless and ringed by black as pictured in the artwork above by Julia Zhuravleva. They have no body hair, but the hair on their head can be platinum, white, or silver.
According to legend, the abducted human children are given to the devil or used to strengthen fairy stock. The return of the original child may be effected by making the changeling laugh or by torturing it; this latter belief was responsible for numerous cases of actual child abuse.
Changelings have red blood like a majority of the other races. Bleeding around others may alert many that the Changeling is not of the same race (should the blood color vary). For this reason, a Changeling will usually choose to take the form of a race with red blood.
according to the rukles for changelings: Languages: Changelings begin play speaking the primary language of their host society and Common. Changelings who have high Intelligence scores can choose from among the following bonus languages: Aklo, Draconic, Dwarven, Elven, Giant, Gnoll, Goblin, and Orc.
Changelings are typically born with female anatomy, but some prefer to maintain male primary and secondary sex characteristics once in control of this ability.
It's not widely known, but one of the few ways one may determine that a person is, in fact, a changeling has to do with their hands. Changelings generally do not have the skill to replicate "life lines," and sometimes even fingerprints.
They have large colorless eyes circled by thick black rings. Their hair color is most commonly a light shade of silver followed by platinum and blonde. In rarer cases, their hair can be pale shades of green, pink, and blue. Similar to elves, changelings lack body and facial hair.
Two changeling parents breed a changeling child. A changeling and a humanoid can mate to produce an offspring with the child having a 50% chance to be of either race. Your question noted that there's a 50/50 chance of getting a changeling if they mate with a human...but it's not just human. It's any humanoid.
Changelings have the unique ability to transform their body to look however they want, provided they keep the same arrangement of limbs they already possess. It is very rare to actually see a changeling in it's 'true' appearance. Even when disguised, changelings can instinctively recognize each other.
Changelings can look like anyone at any given time, but all changelings have a true form. The true form of a changeling is usually a pastel color, ranging from light pinks, purples, greens, yellows, and blues, even greys.
The traditional idea of a changeling is not literally about a transformation. It's about replacement and loss: a human child is replaced with something else that's unwanted. Historically, someone might be accused of being a changeling if their family considered them shameful, deformed, or unwanted.
Obviously, a changeling's appearance can change at any moment, but they do have a true form. Their true skin is usually grey or pale white, and their facial features are blank slates to an unnerving degree. They're typically thin and gaunt, and their hair is usually silvery with an occasional green or pink tint.
Bear in mind that people actually believed in changelings well into the 20th century. Other remedies for a changeling were things like putting it on a hot shovel, leaving it out in the elements overnight, or threatening the suspected elf with torture.
In some cases, the changeling grows up often being very loyal and caring to friends and family despite what they are. They often care more for their friends and family than they do themselves. Most changelings enjoy helping as they get older, but they often feel different, out of place.
The Traveler is the patron of all those who welcome change, in body and in philosophy. Its worshipers include changelings, shifters, doppelgangers, and lycanthropes, who see it as the greatest of gods, but only doppelgangers and lycanthropes routinely revere it.
A changeling can be detected by its huge appetite and the large quantity of water it drink (a bathtub before morning, if left beside it), its large teeth, its fondness for music and its powers of dancing, its unnatural precocity, or from some unguarded remark as to its own age.
Most changelings honor the Traveler, one of the Dark Six. A passer openly worships whatever is the most suitable deity for her chosen disguise. In times of stress or if their cover identities are threatened, most passers will offer silent prayers to the Traveler, the master of deception.
The new child—the changeling—is characterised by unresponsiveness, resistance to physical affection, obstreperousness, inability to express emotion, and unexplained crying and physical changes such as rigidity and deformity. Some are unable to speak.
Even when in humanoid form, Changelings did not need to take in sustenance. Odo noted that he once attempted to eat after assuming humanoid form for the first time, but found the experience not only unsatisfying due to his lack of taste buds, but "messy" for reasons he chose not to elaborate.
As for being tattooed, changelings can definitely be tattooed, and they can just as easily erase the tattoo with a moment's thought.