It's harder to tell if it's properly clean than most people would imagine.
I've had discs before with no visible dirt that didn't read properly (even one I'd just bought that hadn't been played) and they read fine after cleaning.
You've nothing to lose by cleaning it so wipe it gently with a soft lint free cloth and some isopropyl alcohol or methylated spirits. These are safe to use on polycarbonate and won't damage the disc.
Don't get any on the label side though as they can smudge certain types of ink used to print on commercial discs.
It that doesn't work then you may have very small scratches on the disc but there are quite a few methods of dealing with this.
It's surprising how much a very small scratch, that just happens to be in the wrong orientation, can affect the laser.
Oh and I've made the thread title more relevant as anyone searching for this particular topic wouldn't be able to find it otherwise.