Depending on the mood of the fish, I've had a slight twitch in the line, the line shooting off in another direction feeling weightless, the lure feels weightless, the lure feels snagged, but not moving, the lure gets smacked on splash down (once I actually had a fish jump out of the water to inhale it prior to splash down) and lastly, the lure gets smacked with all the subtleties of a runaway freight train. I'm missing some other ways here, but I'm sure you'll experience all these.
I fish weighted and weightless t-rigs on semi-slack line. Cast, let out a little line for a straight fall and maintain a slight bow in the line. You should be able to feel what's going on with the business end.
All the above suggestions for retrieves are very good ones. Once the dog days of summer hit, my most productive retrieve is nothing. Just cast and let it drop. I've waited like a minute dead sticking a weightless rig and made a pattern out of it.
Good luck