My cat seems to be having asthma symptoms and I am taking him to the vet at 5 PM. He hasn't had a complete attack (if thats even what it is) since about 6 AM, hes just been grunting and breathing a little funny. If the vet x-rays him when hes not having symptoms can he still tell if my cat has asthma?
Yes, they look for certain changes in the bronchial tubes – like inflammation. it will be there even if he's not currently coughing. Ideally your cat will cough right in front of the vet 
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A physical exam and a complete history will determine if they take an x-ray (sedation is not always necessary – it depends on the attitude of the cat). They can see changes in the lungs & bronchi. Also, heartworm disease in cats show up more as lung/breathing problems than they do in dogs.