Input lag is a very finicky, subjective kind of thing. The best way to tell is to get the numbers, but beyond that everyone has their own sensitivities to it. Often times, people will aim more poorly on a certain screen (due to input lag) than on others without even realizing it. As for me, I can't directly tell either, but if I'm having trouble aiming, then I usually know what it is. (i.e. I identify light input lag by its negative effects, not by trying to observe it and "catch" it lagging.)
Monitors are actually the best devices for gaming possible. They are almost always built for low latency, and you can focus your money on the important stuff; the video display, without fancy things like tuners, image processing, extraneous ancient inputs (i.e. YPbPr), horrible-quality speakers, and other features mucking into the cost of the device. Some ASUS HDMI monitors do seem to have more input lag than usual though. In the end however, it's up to how strict you want to be on latency; if you want it to be as perfect as possible, use a CRT.