The fuse has got to stay in while testing for 12v on both sides.there are two metal parts on the top of the fuse...one on each side..one for each blade of the fuse.check each side for power.if one side has power and one doesnt the fuse is popec
Or, take the fuse out and check the 2 fuse blades for continuity.
Either way will tell you if the fuse is good or not.