Observing ant hills and seeing Alates at the entrance [especially if it's happening at multiple colonies at once] can be a good way to see if the ants have Alates. It's no guarantee that they will fly that day, but it's a good sign they will soon.
As dermy points out, this is a telling precursor for nuptial flights in many species.
One of the best ways to increase your anting success, especially if you're looking for a certain species, is to 1. know your local ants and 2. be cognizant of their life histories. You're halfway there if you know the colony's location. I encourage people interested in ant keeping to actively collect workers in areas they spend a lot of time. Knowing what species occur somewhere you spend much of your time increases your likelihood of seeing a flight.
The other part is knowing the life history of the species in which you're interested. For example, Tetramorium caespitum queens fly early in the morning in my area. If you know where lots of colonies are, but only search in the evening, you'll likely miss your chance for that species. This is where reading and watching your local colonies is important. Alates of some species sit outside of the nest for a day or so prior to flying; think of it as testing the environment and waiting for just the right conditions. This is usually a good sign for an upcoming flight, but keep in mind some species that exhibit this behavior can also be finicky. For me, this has been the case with a nearby source of Aphaenogaster occidentalis. The colonies I use are reliable, but incredibly picky about conditions. Last year, alates sat outside the nest for one to two days on five occasions over a six or seven week period before they finally flew.
Camponotus usually fly in large swarms, especially in areas where colonies are dense, so it'll be hard to miss if you're checking in frequently. Look for spider webs in the area. During heavy flights, spider webs get littered with alates, which is a sign you missed the flight. Don't let that discourage you if it's Camponotus you're after. They're easy to find behind loose bark of fallen trees soon after flights, and if you wait until winter, you can easily collect queens and workers in diapause this way. Just be responsible and don't destroy all available habitat in an area.