if she's 6-7 months old, she's probably already been in heat or will be very very soon. the problem with that is, she'll attract every other male cat within a fifty mile radius (may be exaggerated, but you get the idea) - so even if YOUR male kitty is actually fixed, she might still get herself pregnant very soon.
if she's in heat, she'll act much the same as your indoor kitty did when in heat, you'll just have to spend some time with her to know

but every cat is different... she might not be yeowly, she might just "present" to you or be extra-cuddly. sometimes their nipples swell or get pink, sometimes not. sometimes you'll notice blood, sometimes not. or if you suddenly see ten strange male cats show up on your property, that's a good hint too.
i would personally recommend getting her fixed ASAP. cats can and are fixed while in heat, though some vets aren't comfortable with it (there's a bit of a higher chance of more bleeding) and won't do it, so that's kind of something that would depend on your local vets. cats can also be fixed at any stage of pregnancy, though again, the later you wait, the higher the risk, the less ethical perhaps, and the fewer vets are willing. but if she's not showing, you're probably okay.
i recommend it sooner rather than later though, because you'll avoid most complications if you can get it done asap: no unwanted kittens, no accidental second pregnancy while she's nursing said kittens and/or having to separate her from kittens to get fixed, no unwanted male cats on your property (read: spraying and fighting) etc etc
