How can you tell that someone is a transsexual?

First, it is probably wrong to even want to know this about other people. They transition to eliminate their birth condition. So if you know, then all the time, effort, and money they put into their transition is wasted. That aside, it depends on who wants to know, what type of transsexual person is involved, and the exact transsexual person.

If you want to know if you have TSism or not, you would pay attention to your feelings and who you believe you are. Remember things that you wanted to do but that your parents or teachers wouldn't let you do, but that members of the other gender were allowed to do. Remember getting punished for dressing up as the other gender. Remember all the things you were deprived of just because of your gender. Then look at your current life to see how stressful and lacking in fulfillment it is, and see if that is related to the past pain.

If you want to know if a woman formerly suffered transsexualism, you would look at things like her height and build, whether she has facial hair, whether she has an Adam's Apple, how wide her pelvis is, how wider her shoulders are, and the size of her hands. There may be differences from other women in the fingers too. The nail beds may be more square (wider) than those of other women, and her right index finger may be noticeably shorter than her middle finger. Her voice may be deeper than that of other women. However, none of these things are 100% reliable, and there may be health-related explanations for these.

If you want to know if a man formerly suffered transsexualism, you would look at things like height and build. His pelvis may be wider than that of other men in relation to his body, and he may be shorter. His voice might be a little higher than that of most men, and he might have a smaller Adam's Apple from starting hormones later than the time of puberty. The rest here may offend some, though not intentionally, and it certainly doesn't apply to all transmen. Some TS men may feel sexually insecure. A TS guy may overcompensate in his masculinity and do things that most men don't do anymore. He may grow a beard, smoke pipes or cigars, get macho tattoos, or work out in a gym. A few may adopt negative male stereotypes in an attempt to be just one of the guys. Sure, many men these days will do a couple of these things, but some TS men may be making up for lost time and/or trying to make sure others know they are not women. Many people attempting gender transition in either direction will start with stereotypes and then relax into a style that is more comfortable and appropriate for them.

Transgenderism is not exactly related, but there are some extra cues in addition to the above for transgendered persons who don't want surgery but who try to pass as the other gender. That would include things like constantly seeking for other "T-girls" to meet, or swearing up and down they are not gay and then promoting gay causes with the remainder of their breath. They may put loyalty to gay men and cross-dressers before any loyalty for persons with transsexualism or other members of the mainstream community.