How can you tell if someone has intimacy issues?

you can't.

What is an intimacy issue?

Answer

Someone with intimacy issues will not be able to form a closeness or an emotional closeness with anyone. They will keep you at arm's length either because they don't know how to be intimate with someone (they didn't have a good role model like parents) or they have been hurt in the past and are too afraid to really allow that kind of closeness in a relationship.