If the tire has tread wear indicators, there should be a mark on the sidewall of the tire, a little arrow or dot or something, to show where it is. When the rubber of the tread is worn down even with the wear indicator bar, it's definitely time to replace.
I replace mine when they get to the point that they don't get as sticky as they should when hot...when it starts trying to spin up or slide easier than it should. Usually mine still look like they have a lot of tread left in the center, but the sides are visibly worn. They usually are tearing or balling a little and the tread voids chunk a little. That's normal. The tearing usually occurs when the weather and road are cold. The balling is normal and necessary...that's just the worn layers of rubber tearing away to expose fresh, sticky rubber and where the edges of the tread voids tear away a little is going to happen when you have tread voids going all the way to the edge of the tire and usually shows up right at the edge of the tire. The wider the void, the more likely it is to occur. It's a trade-off for being able to corner in the wet. The GSXR1000 is pretty hard on tires, though. Soft compound tires are only good for so many heat cycles, plus the outgassing with age makes the rubber harder providing less traction.