How to test and restore the health of a pond?

Toss a match. If the pond burns, there is too much gasoline in there. To be a bit more practical, your water company either has a lab or can tell you what lab they use. Alternatively, check the yellow pages. Your main problem is to prevent continuing pollution of the pond. Cut that out and the pond will eventually clean itself. You can check progress either by having the pond checked for petroleum products or do a general health test by monitoring benthic macroinvertebrates. I'd offer to help but you probably couldn't afford me.

Testing is easy. You can send a water sample off to a laboratory or buy a DIY test kit. If it's public land you might be able to request a test report from your states Department of Natural Resources or the EPA. What you can do about it depends on who the pond belongs to and what type of contamination is present. Worst case the pond might have to be drained and a liner installed.

Look to a local nursery that sells pond liners and treatments they should know of a water testing lab in your area. you take a sample there and they will tell you what you need to add to bring it back to life. as far as the gasoline in the water, not sure about that you might have a enviro nightmare on your hands.