How to tell if i have tendonitis or something similar?

Tendinitis: Typically has pain with movement in the direction of or opposite to that tendons pull. However in severe cases sharp pain even i cues at rest. Rather than being a diagnosis if exclusion for a fracture or arthritis the diagnosis of tendinitis should always be anatocalky specific . If told you have tendinitis then you should know exactly what tendon it is that is inflamed. If not then question this.

You probably can't: However a doctor can. See one if you have significant problems.