How can I find out if my floor is level?

Measure the height between the floor and ceiling over the width of the cabinet in different places. Is there no difference? Then you can assume the floor is straight. If there is a difference, test if it is on the floor by laying a long spirit level or batten on the floor. If this does not yield a perfectly straight result, you can assume that the floor is not level and we advise you to opt for adjustable feet.