I don't want to sound trite, but, it's called match the hatch, as a preferred approach on still waters.
When that is not possible, then searching patterns come out first, like a swimming nymph, caddis or mayfly imitation, followed by perenials like a leech, and when all else fails, a wet-dry or sunken wet will usually produce.
Fly selection varies based on your prior scouting of the type of bugs likely to hang about the lake you are visiting. If you have not done any scouting, you can usually bet that there are mayflies of some sort, very likely caddis at some stage of life, leeches, and if there are cat-tails or similar reeds, bet on dragonflies and damsels being about.
Pete AKA Frenchcreek from Calgary