[MrBasse]

Couldn't you just put some traction wheels of the same diameter on one side of a robot and your mecanum wheels on the other and drive straight? If it turns one way you'll know you have rollers causing loss of forward motion on that side.

For that matter, use your encoders to rotate your wheels a known number of revolutions. Calculate the distance you should travel based on circumference of your wheels and check that against the actual distance traveled.

I just feel like this is a great opportunity to go and try it to figure out the answer. Unless I'm missing something and neither of those would give the answer you are looking for.