There was nothing obvious about it. At no point does Jack explicitly state in ME2 that it isn't her thing.
If memory serves I believe she does indeed say that she isn't interested in women in one way or another. You'll have to forgive me for not remembering the exact quote, but I'm sure someone her could help you with that.
There are many women in real life and in fiction that have engaged in sexual activities that involved the same sex in one form or another who do not identify as lesbian or bisexual. If Jack did have those interests towards female Shepard, one would assume it would have naturally come up in one or more conversations through out the game in the form of flirtation... or simply because of the fact that Shepard is showing some form of interest in her, romantic or otherwise.
However, unlike with male Shepard, the casual sex line which leads to *shock* casual sex, is absent for female Shepard, and the topic of romance never goes beyond her recalling that single event from her past. It was the combination of all these things made it clear to me that Jack was not interested in a same sex relationship, and if that wasn't made clear enough enough, I believe she spells it out for you after her personal quest if you persist.