There are a few ways to go about this, these are the main techniques I use in the field. these are for the UK but would be transferable I imagine.
Note: You will need to check the law relating to disturbance of animals and habitat before doing any of these. I have no idea what your state legislation allowswith regard to this.
Daylight by eye
If you are patient/lucky and depending on the species you may well be able to tell what's in there by just watching. A visit first will help determine whether the water body is suitable habitat.
Torching
After dark (obviously) This is probably the easiest to perform but until you get your eye in for identification it can be tricky. Also if you want to view close up it's not so great. For a really good look after dark you need a torch with at least 1 million candle-power. The more turbid the water the more difficult this will be.
Netting
Use a net which is not too coarse in order to prevent damage to the animals. This is a bit of a balancing act because if it's too fine it will slow the nets progress through the water. I use a 2-3mm mesh. It's probably worth having a small aquarium net on hand to help with closer examination and don't forget some examination trays. A cheap white cat litter box works well. Netting is however highly destructive to flora and the pond in general especially if done in a heavy handed manner.
Trapping
Trapping is probably the most effective method but require considerably more effort and commitment. The simplest trap to construct is a bottle trap. Take a 2 litre bottle, round works best, cut off the top 3rd and flip it round placing the top section inside the bottom essentially creating a funnel. You can then either pass a garden cane through the bottle at a 45 degree angle or attach a tether to the bottle with a weight at the other end.
Now the important bit when you put them in the pond overnight leave an air bubble at the top of the bottle to create some air exchange. The warmer the weather the less dissolved oxygen is in the water. Don't leave the traps out for any longer than 7-8 hours when the weather is warm. In cooler months this can go to up to 12.
There are other traps but that's probably the simplest.
Remember keep disturbance and handling to a minimum.
Hope that helps.
Happy herping.
