The only method I know of that works is flying interceptors over them.
That said, the game's default configuration is to auto-reveal alien bases 5 days after they are built, so I wouldn't worry overly much about trying to find them. 5 days isn't all that much time in the scheme of things.
Note that if your concern is that you want to raid alien bases but aren't seeing any, the problem is not that you haven't found them, but that you're too good at shooting down UFOs. I find that once I have 3 bases up and running with full radars, there isn't a lot of room left for aliens to land where I won't see them. It does still happen, but it only seems to happen once every few months. Since a successful raid on an alien base yields the nice Base Upgrade research option, you might consider letting some of the larger UFOs through if you get to December or January and still haven't seen a base.
Once you do the appropriate research and build the Quantum Cryptology Center, you can see what mission a UFO is on and specifically let those which are trying to build an alien base land, should you wish to raid more of them. Unfortunately, to gain the ability to research this upgrade, you must first sack an alien base, so there's a bit of a chicken and egg problem here if you simply aren't ever seeing them appear.