It doesn't matter what she is, but it matters how she perceives your acceptance.
Springing the question is right out, because it IS none of your business. You wouldn't want to embarrass the kid.
You need to avoid innocent negative stereotyping of lezbos from the common male perspective, like making lewd sexual comments if two chicks are making out on television. Example, if a good looking man and a good looking woman kissed on television, you wouldn't say "Oh god, hottt!!" But many males do with the lezbo kiss.
So I would just avoid the subject altogether, and not offer any subtle comments like "I'm cool with that" and instead, opt for neutral ground. Nothingness. That is the only true path to real acceptance, as far as I'm concerned.
People get fucked up with acceptance, and think they have to do positive things, like support rights and understand the culture, which often fails miserably. From a perspective of neutrality, its like being accepting of a generic looking house among others, or blue sky, or who is in second place in NASCAR standings without caring about NASCAR at all. It doesn't matter.
Peoples senses are extremely attuned towards subtly shifted eyes, smirks, and lightly laid comments and white lies when they are "guilty" of something. (don't get hung up on the word 'guilty', Rill) You've gotta practice flushing all of that out of your system, and replacing it with "so what". If she says "Indy, this is my friend Cathy", thats when it comes into play. If things were different, you'd say "hey" and think nothing of it. Thats the only way to be.
Eventually, it'll just be an assumed thing, nobody has to come dancing out of the closet. Maybe one day, she'll say "...but you always knew that, its not like it was a big secret." and you can say "yep". But if between now and then, you attempt to broach the subject and make things awkward, you will be remembered as a difficult part of the process.