I'd say the biggest artifact's that I was noticing was "waving" on the sides of prints that are aligned with the X and Y axis. Say you print a cube that's parallel to the X axis.. if you have any slop or wear on the belts it can cause waving effects on the outside perimeters of the print. Additionally, when printing at small layer heights (<0.1mm) you will notice layers not lining up especially on long straight runs in the X or Y direction. The misalignment is very subtle, but can be felt if you run your finger nail across the part. This isn't a picture of my print, but an example of what you might see.
Ultimaker recommends that there should be no more than 20% of the belt tooth missing, if you have a newer machine that doesn't have as many hours maybe you can compare.
Not trying to sell you on getting new belts, if its working and you're happy with the prints I wouldn't worry about it. The risk is high if a belt fails so I always try to stay ahead of items like that.