I did a wheel bearing on my W124 this weekend - the same tight tolerance applies to the end float or play. I used a mitutoyo gauge;

http://www.mitutoyo.co.uk/MitProd/mtopr.nsf/UNIDS/F255B23AEA15F53A80256953002EAC76!Opendocument

Specifically, the 1013F

However, any gauge that can measure down to 0.01mm should be OK - you shouldn't need to spend a fortune.