Step back a few paces then eyeball the two casters to see if they are both the same or the bad one is twisted back.
If the bearing is gone you can wiggle the shaft back & forth once the wheel is off the ground .
Rarely does this happen with a new mower.
However under the cap is a big nut which does come loose and that will be evident by being able to move the caster up and down.
The cap is SOB to get off however it is just a light press fit ( that is hydarulic press ) so some work with a big blades screwdriver and a mallet usually will pop it off
With customers mowers , I file a slot across the top so next time there is an easy place to put a punch