I have a 3 year old bullmastiff that i got from BARK rescue and i was told she was not speyed, i have been waiting for her to come in season for a year now so that I can breed her with my bullmastiff male, but she shows no signs of coming into season. Im starting to wonder if maybe she is speyed after all? is there any way to tell for sure? is it normal for a bullmastiff bitch to be 3 years old and never had a season?

It is highly unusual for a rescue society to release papers and registration with their dogs so maybe it is just as well if she is spayed.  Without her pedigree you will be totally lacking in the necessary information to confirm the lucky coincidence that she is a suitable mate for your dog.  Has she had the necessary health screening recommended for the breed with good results, has she been judged a good example of her breed either by show judges or informally by people experienced in the breed.
Without at least a stab at doing all this properly you are laying yourself wide open for a compensation claim for any puppy that develops a problem in the future.

Dogs from rescue are frequently neutered before rehoming if they're old enough, so I'd guess she's been spayed already.

I hope BARK knows about this, rescue dogs should never be bred from as you dont have their full history :(