Tools:DRC, clearance
I use the iteadstudio DRU file for checking stuff.
Put these in the Eagle/dru and eagle/cam folders.
Then select
Tools:DRC:Load, click itead_rule.dru, go from there.
Your choice of board house may have their own files for checking the layout.
ITeadstudio_CAM.cam (10.3 KB)
ITead_rule.dru (2.35 KB)