The horrible thing about the TPS is that they don't tend to just peg out and stop working, they play silly buggers for a while until you just know something ain't right.
The obvious thing for starters is making sure the needle's sitting on the 5K position on the rev counter.
If it's not, then adjust it until it is but then keep doing quick checks on it every day if the bike doesn't quite feel like it should do.
If it keeps moving position, or stays on 10K or 0K, then it's on its way out.
You won't tend to notice it too much over 3000rpm, although it still won't be right, but under 3000rpm it will feel very jerky and you will need to use the clutch lever much more than normal to keep some kind of proper control over it.
It's possible you may suffer starting problems too, and also the tick over may not appear to be very stable either.
Unfortunately a dodgy TPS can also resemble a badly unbalanced set of carbs as well so keeping an eye on what the RPM needle is doing is important.
If that is showing as being stable on the 5K mark then it could be that the carbs just need syncing. If this is the case then you need to make sure your air filter isn't in a bad way either as any stuffed up airways can give false readings on balancing.
If you do think you may need another TPS unit, don't bother with a used/second hand one. These bikes are at least 14 years old now so, as i learned twice already, if you buy an electrical part that's 14+ years old with unknown mileage on it then it's just waiting to die as well so just buy a new replacement part and be done with it.
These will work on all models.
http://www.ebay.co.uk/itm/Yamaha-FZS-600-H-Fazer-2002-Tourmax-Throttle-Position-Sensor-/331848912138?hash=item4d43bc8d0a:g:~wYAAOSwQNRXK4wG