Almost everyone has a set of drill bits around. Gently twist an appropriately sized bit in the discharge nipple to see if sand or crud is blocking the hole. If that doesn't do it you need to check the impeller. I had one engine
that seemed to need to have this done on a regular basis and kept the drill bit in my fishing
Once I had to take an 8 HP I purchased apart about six times to get it to work
. It turned out bits of the prior failed impeller had wedged in every conceivable location. Each time I thought I had it fixed, and the whole experience made me glad I was never employed as a urologist.
Another way to do it is to take a piece of windshield wiper hose from AutoZone and put it over the nipple without the engine running, then blow through the tubing to see if you can hear the air going down the tube. Don't suck on the tube with the engine running as you will be inhaling exhaust
gases (yes I have seen people do this.)