For one, the serial number on the tanks was a weld bead on several areas of the tank. The number could be D1234 or L5678 (D=Detroit Army Tank Plant, L=Lima, Ohio tank plant). A heavy armor variant included the letter U for Uranium after the serial number, i.e. D1234U or L5678U.
The heavy was also when they added the "anti-sleeping bag mines" to the turret ammo blast panels. The original M1A1s had flat panels. HA and afterwards had three raised, circular lift points that were intended for a never-used quick ammo resupply ability. After several years, these vanish and reappear based on if the original tank had them during rebuild periods.
There were also some access panels added to the back deck and some weld differences on the turret forward edge of the roof based on where the DU armor was added.
We also had the one piece T-156 track blocks on the originals and the HAs got the newer T-158 tracks, but older tanks could have received newer tracks later in life so this isn't to be used as a positive visual ID.
I know the regular non-HA tanks I was turning in in August of 1990 that were supposed to go to a POMCUS site in Italy, but when I got to Bremerhaven, they took the entire battalion of tanks plus the two FSB float tanks (60 tanks total) signed for them without inspection, put them on a boat and sent them to Saudi.
Having turned in a company of M60A3TTS tanks just a year before, I was expecting a battle turning in the M1A1s, but the Iraqi invasion of Kuwait made them take them as quick as possible. As I understand it, the former 5-77 Armor tanks I dropped off at the port were the first Abrams tanks to arrive in Saudi. I swear I saw tanks being offloaded on CNN that still had our various company logos on the front slopes of the turrets.