You mean you have an empty entity error or 'solid entity has no brushes' error or something similar? If so:
Press alt+p and see which entities are listed as empty (let's say you have 'the entity func_buyzone is empty').
Go to map->entity report and filter brush entities only. Then select the first of the entities that were listed in the alt+p (in this case func_buyzone). Now press Go to. If it selects something in the map, move on to the next func_buyzone. If it doesn't select anything press delete.
...and that's the bottom line because Mate de Vita said so.