How do I know if my blister needs popping?

Most don't. Blisters form as a protection to the raw and injured skin underneath. Most blisters technically do not need popping. Sometimes they will be drained as they are large and painful and impede normal movement of a body part...Even then they are drained so the bliser skin still lies flat on the underneath skin to protect it. They usually "pop" on their own through normal body movement.
Protect it. An unpopped blister represents a closed sterile environment. Yes on occasion one can become infected then it is time to open it, however the risk of infection in a dressed open wound is still higher than that of an intact blister. The blister will gradually resolve with time.