I guess you could try to intercept some rare cult missions that do show up on the globe, or try to tail the occasional UFO in hopes of getting the landing site. Your interceptors are likely to be pretty anemic, though, and trying to assault a UFO with a carful of agents isn't all that much better. Unless it's a moon lander, which might be doable.
I guess the more important bit is not to forget to build a radar network before the invasion starts, because it takes time and, more importantly, money.
A second HQ won't help much with radar coverage, since the detection chance is as low as it gets. It's much more useful as an extra lab, and overpriced even then.