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You'll never be able to identify a trans person through physical attributes alone. People's bodies are incredibly diverse, and a single physical characteristic won't say much about a person one way or the other. Moreover, genital reconstructive surgery has come such a long way that many who undergo it (not all trans people do) are indistinguishable from those who haven't. You will likely offend them either way. If the person you're talking to is trans, they may not have been ready to tell you. If they're not, and if they have negative feelings towards trans people, they'll feel insulted. Odds are, asking them is one-way ticket to them doing one of two things: Worst case? Throwing a drink in your face. Best case? Just walking away. Disclosing can be very painful and even dangerous for them. This is their information to tell, when they feel safe enough with you. In short, they aren't required to tell you, nor should they have to if they don't want to. At any rate, they will likely tell you before intimate contact is had. If you're wondering if your date is trans, you might be harboring some negative stereotypes in your mind that are not based in reality or you don't want to be mixed up with a trans person. Moreover, your reaction to your date if they are trans, based on your misunderstanding and bias, can be devastating to someone who already faces societal rejection and abuse. If they are trans, they are no less of a person and they should be treated the same way. If you find yourself dealing with fear over finding out your date is trans, read How to Deal With Transphobia and How to Respect a Transgender Person. It may help you understand what they're going through and how they feel. There are many other things to worry about that actually warrant worry, such as STDs. If the person is trans, wait for them to tell you. If everything else has gone well up to that point, you can cross that bridge when you get there. What's the problem if you're just in the dark for right now? If things progress, you will eventually find out. At that point, one of two things will happen: the relationship will continue or it won't. It is not the end of the world. Simply enjoy the person for right now; if it pans out, great. If it doesn't, that's okay too. There are many happy endings that began with a date where one person did not know the other person was trans: It Happened to Me: I Told My Boyfriend I Was Born a Boy and Falling in Love with a Transgender Man are two prominent examples. Sexuality is fluid. You may find yourself attracted to this person despite their assigned sex. That is normal and absolutely fine. Listen to your emotions and how this person makes you feel. You may find that they're such a good person it doesn't matter.However, it's important to remember that transgender people are their gender. They are not "men pretending to be women", "women pretending to be men", or anything else. They can have any physical characteristics, and not all have the ones of their assigned gender.
Be aware that there are no clear-cut physical signs that you can rely on. Avoid asking your date if they are transgender. Examine your own bias. Stop worrying and relax. Keep an open mind.