Make it clear that you're transgender (one way or another). Watch how they react. If they reveal they're totally unaccepting of trans people, that's actually a good thing. Now you know, up front, where you stand with them. You can move on and not invest anything more in that relationship.
It's worse to beat around the bush, try to hide it, and not come out until long after you've invested a lot into the relationship. That puts yourself in a position where you have more to lose.
Unfortunately, it's just a fact of life that there are a LOT of transphobic people out there. You can't waste energy getting worked up over them.
If they're sitting on the fence, feel free to answer any questions they may have. You never know when polite answers might bring someone around. People often have a big knowledge gap when it comes to trans issues. But if they've had time to mull it over and are still pretty hostile, then that relationship may not have a future.
Situations where this is the preferred strategy:
You have a lot more to lose in this situation. If you guess wrong, the relationship is severed, and you've lost the time/effort you've spent on the relationship.
However there are situations that you don't have a choice.
You have a choice about what boss you work for. But you usually don't have a choice about who your coworkers are, unless you get to meet all of them during the interview process. (often all but impossible, since there are an extended number of people you have to interact with to get your job done)
There are some other relationships of necessity, rather than relationships of convenience, where you have less discretion in forming the relationship.
The closet is a maddening place. Self-deception and self-loathing lies in that direction. The risk of suicide is demonstrably higher.
You never know when you might change your mind and decide to come out. If you're in the middle of a long-term relationship when this happens, you're certain cause waves, to put it mildly. All of a sudden, they realize they're dating a very different person than they originally thought they were. They may eventually decide that they're compatible with the real you (if they're attracted to masculinity as much as femininity, for instance, or if you ultimately realize you're pansexual and they're able to rid themselves of internalized homophobia). But even if the story has a happy ending, they still have to confront the fact that their partner is suddenly very different, and that their partner has been lying to them all along.
I personally think it's extremely irresponsible to enter a long-term relationship if you have questions like "am I gay or not?" or "am I transgender or not?" hanging in the air. Figure yourself out FIRST, so that you can enter a long-term relationship with honesty and a clear conscience.
You MUST be honest with yourself, so you can be honest with your partner.