I bought my 06 600RR used and there are a couple minor upgrades the original owner did but I am just curious to know if there is a way you can tell if the sprocket has been changed?

Do you count teeth or is there a mark or something you can look for?

 

Count the teeth on the sprockets, compare to the number of teeth on an OEM sprocket. Smaller front/bigger back means it has been downgeared.

The rear could also be made of aluminium which wears quicker, costs more and is really quite pointless on a road bike. Test it with a magnet.

Finally, someone could have fitted a thinner gauge chain. Again a pointless mod for a road bike but the chain pitch can be narrower to save weight at the expense of chain life.