How can I tell if I'm allergic to clindamycin?

Several ways. The preferred manner of testing depends on the history and the type of reaction. Skin testing can be performed, but there are no validated commercial extracts for clindamycin. If necessary, an oral challenge can be done.
No testing. There are no testing protocols to diagnose allergy to clindamycin. Therefore, the only way to establish this diagnosis is via oral challenge.