Every species of insect has its own story. You have lumped all flies under one heading and it isn't like that. However, in general, flies are pollinators like bees. I would expect earwigs, caterpillars, weevils and beetles before flies for overall leaf eating. In general, I've never seen a typical fly eat my plants. Further more, larvae of some species are attracted to host plants. If your damage is specific to a type of plant, I would look into insects that are partial to the plant and see if that is what you are looking at and the damage done is consistent. The adult of these larvae may look like what you would call a fly, even though in reality, it is classified as something else.
If you have a digital camera with a macro option, you could always post pictures of what you have on the pests and diseases forum here at GW. Bugguide.Net and WhatsthatBug.com are good sources too. Once you think you've identified your bug, you can google it and see what you find out about it.