sometimes there are laws that protect you. certain countries protect you, certain states/provinces/regions protect you, certain cities and municipalities protect you...in theory.
assume the worst, because either way the deck's stacked against most trans people when it comes to actually proving discrimination occurs if and when it does. the optimal arrangement, to be honest, is to take time off of work, change your documentation, transition socially, whatever you want and need to do, before starting another job. that said, financially that might not always be an option. if you're willing to be out with your trans status, there's nothing preventing you from showing up to an interview dressed however the hell you want, as long as you're prepared to discuss that potential incongruity with your prospective employer.
that said, what i would advise is that if you're going to be out as trans in the workplace, be out when you get there as opposed to starting the job and subsequently transitioning. i've been in places and times where i've had to come out and transition in front of the same people, and it's a pretty miserable feeling; whenever i've been there i've wanted to get out and cut all ties as quickly as possible. of course it's possible that you're not me and you would feel fine and ok with that, but i know that's a common sentiment, so i thought i'd at least say that.
