It appears that there's no effective method of detecting leech infestation without physically seeing them. Leeches are usually found in the muck at the bottom of a pond under rocks and any other debris they can find.
However, there are some things that you can look at in concluding the presence of these creatures.
You can unintentionally boost leech numbers by causing excessive plant growth or nutrient loading by, for example, using soap, shampoo, or fertilizer in or near the lake.
Dumping anything (fish guts, garbage, etc.) into the water can translate into either food or an extra hiding spot.
Also, an overpopulation of leeches in a pond can cause harm to fish and other aquatic life living in the pond. So, a fish population that is too low means an abundance of leeches in your pond.
If uncertain, don't be rest assured that they don't inhabit a pond/lake just because you don't see one. While they are generally nocturnal creatures, during hot summer days, leeches are particularly attracted to water disturbances that create vibrations, such as animals swimming by or splashes created by people wading or swimming in the water.
Leeches are excellent swimmers. They come out of their hiding spot and attach themselves to their host animal (which sometimes ends up being a person).
Get more info about leeches at Nhlakes.mylaketown.com.
Tip! Get leeches out of your pond with Pondplace.com's leech control tips.