Problem

You dont.

Blank DVD's are speeed rated so if it says 16x then most good burner software knows this and will burn at that speed
If you get a 23x DVD and try and burn it on a 20x dvd it may be more prone to failure.
If you had an 8x dvd and tried to burn it sater than that then it too is more likely to fail.

CD's are a different matter and ca be slowed down but should not overspeed them or once again they may be prone to failure.

you don't,
for reading the computer uses the fastest speed it sees fit.

for recording it will default to the fastest speed the media allows. but most applicaitons that you use for recording will let you choose slower if you want.