Well first off, before you have intercourse, you should ask him. Now this is a very serious matter. If he wants to have sex with you and he has AIDS but he hasn't brought the subject up, then he is not a good partner to be with, because you have to share that fact with each other so you don't risk it. If you are not sure if you're boyfriend has AIDS, then you should go to your local GP or family clinic and get a blood test to be sure.
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The only way for someone to find out if they have HIV or AIDS is to get tested. If you want to know if your boyfriend has HIV, you should ask him; you may also suggest that you go get tested for all STIs -- including HIV -- together.
If s/he already has AIDS you will know it since AIDS is a syndrome, i.e. a bundle of symptoms which the person has. A HIV+ person on the other hand you cannot know unless s/he produces testing certificates(or you trust what s/he is saying).
A negative antibody test is not reliable until it has been repeated while the person is in celibacy or having one partner who is HIV free.
A negative virus test is on the other hand reliable.