You can probably just slide the lid up the discharge and vent enough to peek in with a flashlight. If not, you'll have to remove the coupler or check valve on the discharge and possibly a coupler on the vent.
The pump should look like a sump pump. Look for the float getting lodged up against the discharge or the vent.
You should have someone flush the toilet a couple times while you observe how it ejects.
If the float appears unobstructed and the tank empties completely - just slowly, check the gate valve (should be fully open) and the check valve (should swing freely). To inspect the check valve, you'd have to unscrew it from the discharge. But if/when you do THAT, be careful you don't get a column of water (btn the gate and the check) flushing down on you.
If the float IS obstructed, then rotate the pump a little around the discharge until it's in a position that lets the float swing freely in all directions.
