There are a few differences, early TD18 had the oil and fuel filters grouped together at the engine font left side, 3 filters in a row.
TD18A seperated the filters with 2 oil at end front and 2 fuel at engine rear, still left side.
181 & 182 series both had boxed track frames, an oil bath air filter, 181 still has 2 exhaust, single pipe is 182.
I have seen some late TD18A's that had boxed track frames but not the oil bath.....should be a rough guide to identifing.
Good luck
While IH didn't go to any trouble to match engine serial # to the machine they are usually close enough if the engine has never beeen changed, look for a flat casting boss on the side of the block with a stamped number.....