As said above you will need to remove the vales for inspection.
If they are not visibly bent by eye or with a machinist square you will need some tools like this.
A indexing collet punch grinder
and a Height Guage with Dial Indicator attachment
A extremely flat surface will also be needed, A granite inspection table is the best but something level and flat should work.
The Valve will go in the Indexing Holder. Set the height gauge on the valve. and rotate the holder. This will give you a read out on the indicator and you can see if the valve is bent. You will see the dial indicator read from a positive to a negative number back and so on.
Or you will see very little movement on the gauge.
I have no idea what type of tolerance Yamaha has on the Valves but almost all dial indicators read out to .ooo1 of an inch
If you have any Tool and Die or machinist friends that will be the quickest way to get access to those tools.
Hope this help.