How do you tell the difference on the surface?  Is it even possible without asking?

I have two or three FTM/Butch types who come into my store all the time.  I REALLY like them as people and feel a close kinship with them.  I interact with them and they have androgynous names so I never really ask what gender they are feeling it inappropriate.  If she is a butch, she might get offended, and if he is an FTM, he might feel anxious about not passing.  If they ARE FTMs, I'd love to tell them how much I understand their journey, and share with them my own.

I have always felt closer to the FTMs I knew IRL than my MTF friends for yet unknown reasons.  I also tend to get along famously with cis-gendered lesbians (strangely enough) but cis-gendered gay guys and I don't click all that well generally.

I hope none of what I said comes across badly.  I just have this very strong desire to bond with them and share my admiration with them for their trials and struggle, but I don't know how to approach them (if I even should).

EDIT: I'm really under the weather right now so sorry if the message is muddled.

I don't think there's a way. If you get along well with FTMs and lesbians and such, you're probably the best judge and if you can't figure it out then I don't know.

The direct approach can work wonders sometimes. Next time you see one of these folks, you can say, "I want to give you the best possible service that I can. I don't want to be nosy or offensive, but do you prefer male or female pronouns?"

If you choose this approach, try it on the butchest one. If it looks like s/he binds, that's usually, but not always, a good sign (as Kvall points out).

Only one possible down side that I can think of: while an FTM is almost sure to be pleased and even flattered, certain butches won't be thrilled to hear that you're wondering if they're male.

One of my profs asked me this years ago when he was writing me a letter of rec for grad school, and I was flattered as hell. And one of my students asked me a few years back, and I was again very glad to be asked. But then, I'm an FTM...

I'm pretty good at judging if a person is trans or not without asking...but that's cause I'm trans myself and have learned to pick out things. Not all ftms pack. Most bind. Personally, I would feel like I failed at passing if someone came up to me and asked if I was ftm. Bringing up the trans movie might work, or maybe bring up some pride event in the area.

Do they come in together? It'd probably be safe to ask which pronouns they prefer if they do. That way they won't feel like it's a mark on their individual passability so much.

I agree with nero... just ask them which pronouns they prefer. If they are friendly they shouldnt be offended or anything :]

There's (well, there was) this person in my university who to me looked like someone that was transitioning from female to male.  Extremely masculine in appearance & manner of dress, very subtle facial hair, female voice, & an obvious pear-shaped female body.  Some of the female students had Jokingly said that she could even pee standing up although I'd never seen her do that myself.

Anyway, I found out by a college friend who was dating her that this person is a gay cisgendered female & ID's as a "boi" that apparently means a very butch lesbian.  Until then I'd never heard of that term before.  The thing is that she isn't a trans man & isn't transitioning to anything.

Imo you can't tell the difference.  I couldn't.  The only way to find out is by asking but that's a tad risky too since people can easily get offended.