Ideally a 2 person job to get an exact pond level.Works especially well if you have a large excavation and the use of a spirit level and straight edge would be difficult.Place a funnel into each end of a garden hose. Stand either end of the excavation.
One of you pour water into the funnel until it shows in the funnel. Whoever is holding the funnel lowest will be first to see the water. The other person lowers there funnel till the water shows. There is now (obviously) an exact level from the water level in one funnel to the other. Hold one end stationary at the edge of the pond site. place a stick into the ground relevant to the water level.
The other person can now move around the site placing guide sticks relevant to the water level in the funnel. There you are, you now should have lots of sticks all around the site, the tops of indicating the exact level of the pond...more accurate than a spirit level.