You can buy a gizmo at B&Q which has a laser line to show if a picture or hanging item is straight - or use my idiotproof method, measure straight up from the mantelshelf and make two marks at the same distance up, to put the fixings in.
I have a huge heavy Indian wood mirror over my fireplace downstairs, I was so worried about it falling that it stands with its base on the mantelpiece, with coach bolts cemented into the wall holding up the rest of it!