I don't know if I have a cat allergy, and I don't want to adopt a real cat to test myself, what else can I to to test cat allergy?
The most conclusive way to discover if you have an allergy or not is to ask your doctor for an allergy test. There are two types, skin based and blood based. The skin based one, they will prick your skin with the suspected allergen, and see if you react. With blood based, they'll draw a blood sample and test that.
Free ways to try to test it out, is to visit some cats. If you have a friend with cats, or shelters, or even if you live in a location with one of those cat cafes, go there and spend some time.
Keep in mind though that your observations at these locations might not tell the whole story. For instance, shelters often keep their cats in an area with concrete floors and unfurnished cages. Theses types of surfaces do not retain dander, and so for someone with mild allergies, they might not be triggered there, but in a home, you could be due to carpets and furniture capturing the dander much more. It's also possible that there might be another allergen present in these locations that happens to trigger a reaction.
Unfortunately, allergies can also worsen over time, so there is a slight risk that even if you show no symptoms now, later on you develop them. If you suffer from asthma or tend to suffer from a lot of other allergies, it might be better to not get a cat even if you don't show symptoms now.
If you don't have access to a friend's cat, you can always visit your local animal shelter and visit the cats there. Cat allergies are usually allergies to the (cat's) saliva on cat fur. If you bury your face in a cat's fur and don't have an allergic reaction, you are probably pretty safe.
A few ideas to add to Kai's excellent answer:
Spend the night at the house of a friend who has cats. Preferably a friend who isn't a clean freak. If you are allergic, you'll soon find out.
Consider fostering a cat. Check with shelters in the area to see if they need foster homes. A typical scenario might be a cat who needs socialisation in order to make a good pet; your job would be to help the cat overcome its fear of humans. After a period of time, the cat goes back to the shelter (or you can adopt it if you want). If you do have a cat allergy, it's probably a mild one (or else you'd already know about it). If necessary, you could return the cat early, but you could also just take antihistamines while the cat is with you.
Or just adopt a cat on a trial basis. All good shelters will take a pet back within a reasonable period of time. I just adopted two cats a week ago (yay!) and the form I signed said "If for any reason after I adopt a cat I am unable to keep it, I will return it to the after prior arrangements."
Allergy tests are useful, but they can give false positives. I had allergy tests done years ago, and in addition to a strong allergy dust, it indicated I have a mild allergy to cats and some kinds of grass. I do indeed have a dust allergy, but I've had cats most of my life and have never had any reaction to them. Nor have I ever had any problems with grasses of any kind.