optimized media really work?

Hi,
It's not the first time that i have the impression, that after generating otimized media, checking that they have been generated and checking "use optimized media if available" , they are still not being used.
How could I check, if the optimized media are really used instead of the original media?
Thanks a lot in advance!
Thomas
It's not the first time that i have the impression, that after generating otimized media, checking that they have been generated and checking "use optimized media if available" , they are still not being used.
How could I check, if the optimized media are really used instead of the original media?
Thanks a lot in advance!
Thomas