Doctors have told my husband from birth that he is deathly allergic to asprin, how will emt know not to give?

Wear Med Alert Tag. He should get a med alert bracelet or necklace and wear it at all times. Emt's are trained to check for these items.
Medic Alert Bracelet. Hopefully your husband will be able to tell the ems providers of his allergies, but if he is unable to relay that information, make sure that your family is well aware of his allergies. Bracelets called "medic alert" also give ems vital information such as allergies and medical conditions and may be appropriate for his Aspirin allergy.
Also. I agree with above. It is also important to keep a list of medical problems, medications with doses, and allergies at all times. This is very helpful when someone arrives in the er to know what we are dealing with and what we can and can't give.