Judging by the age of your girl, I would say that the chances of her being in heat are pretty good. Normally this happens at around the 6 month mark.
As for symptoms, I do know that they can vary from dog to dog. Swelling and/or a bloody discharge can be symptoms.
Here is a link that I brought up by doing a search. It may help you get some more info.
There is one more thing that I wanted to add and I'm sure the breeders here will back this up.
If in fact your girl is in heat, you should not let her outside unsupervised. In fact, your best bet is to keep her on a leash by your side at all times. I know that you said your yard is fenced in, but if a loose intact male catches the scent of your girl that fence will do little to nothing to keep him out of the yard. I'm sure you wouldn't want to end up with an unexpected surprise!!
I hope I helped!!
=== My Edit ===
Here's another link with a lot of info about what it's like and what should be done while a bitch is in heat.
Estrus Detection and Management in Breeding Bitches