I had a little bit of a problem with this visually as well. Like a lot of other things, you need to rely on feel as much as you do your eyes. The more you experience slide lock, the more you'll learn how it feels.
You can practice this sometime by either filling up your mag with very few BBs (but keep it random), or by using your thumb to push the slide release up. When you push the slide release up, it will hold the slide in place, even if there's BBs in the mag. You can alternate firing a shot and letting the slide go back into battery, and then fire a second shot and have it lock back, so you can compare the feelings.