Okay, here's how to tell apart an M16A1 with A2 handguards and an M16A2/A3.
This is the M16A1 with A2 handguards.
This is an M16A2 (same as A3, A3 is just full-auto instead of burst fire). Note how the gun has different rear sights (the addition of the windage sights), and has a shell deflector behind the ejection port. Also note how the A2/A3 has a heavier barrel.