I ride a Raptor and not sure when or if it needs an adjustment? It runs fine.

from my experence they dont need them often at all, once every 1-2 years. if its becomes really tight they will start to make alot of clicking from what i hear. and on the 700s adjusting them is as easy as it gets just need a set of feeler gauges.

Getting hard to start is one sign, a loud valve train or noisy valve train is another. This can be the clicking that 1904dunegoon is talking about or just alot of racket coming from the top of the engine. It's hard to tell because the noise starts very gradually as the valve seats wear.

all of the above but I have also been that low speed lunging or sputtering that resembles a fuel problem can be a less common sign.

the factory sets some specifications very close and over board in my opinion. the 660's dont beat the heads in as bad as 450's. but you can tell by loud valve train, and if the bike does not like to idle... had a customer think he was fighting a fuel problem, and realized he never had the valves adjusted with over 100+ hours on his 450

This is how I knew...

Is that the new "turbo" head mod for four strokes?

Yamaha 660 5 valve motors have Sodium filled valves in them and they will stretch faster than other valves. If run hard they will break just like posted in post #6. Valve checking and adjustments should be done often. I check the valves in all our Honda 450s every 3 rides. It's a tale tell sign of what the motor is doing and how it is wearing. Ti valves will beat into seats faster then stainless valves will simply because the Ti valves cup, and the OEM powdered metal seats are not the material that they should be run on. Ti valves should be inspected often, very often. Any kind of dirt or contaminents on the seat will ruin the valve and wear the coating that many OEM valves have. Once that happens it's only a matter of time before it needs new valves and to have the seats cut. Keeping the valves in spec will limit valve train noise (loose valves), and hard starting/tune issues (tight valves).

Not very often do valves loosen. As valves wear into the seats the clearance between the shim/bucket, or rocker arm decrease. Preventative maintenence, otherwise you will pay for it much sooner than need be.