It is almost impossible to just look at her and know if she is pregnant, unless of course she is ready to pop.
I very much so agree that you could have a much larger vet emergency if she is in fact pregnant and has complication during delivery, you could possibly lose both mare and foal.
Horses cost money, constantly, when you have a horse you have to assume that responsibility and give them the adequate care to ensure nothing bigger happens.
A simple ultrasound or a palpation is all that is needed from a vet and typically isn't an outrageous expense. It's better to be safe then sorry. Another thing is if she is pregnant she is going to need a bit more in her care, different worming schedule, shots, even different feed to ensure the foal and mare are healthy and safe.
Good luck to you and the mare.