GT40 heads have the 3 bars on the front and GT40P heads have 4 bars. Also if you look at the lower head bolt by the number 1 or 8 cylinder you would see GT for GT40 heads and GTP for GT40P heads cast into the head.
E7 heads can only be positively identified by removing the head and flipping it over. It will have the letters E7TE cast into the exhaust side of the bottom of the head.
If you don't identify the heads as being either GT40 or GT40P heads then there's about a 98% chance that they're E7 heads. Some people did put non E7 heads on when working on the engine but it's not seen very much.