How to measure water pump pressure in a spa heating system?
Another solution might be to just buy and install a new pump. If that solves it, you're golden. If not, you now have a spare for when the pump does eventually die.

Obvious caution: Electricity and water. Be VERY sure of what you're doing when you make the connections and seal them up.

(I presume you've already checked the obvious -- made sure all valves are fully open, that any flexible pipes aren't kinked, and so on.)