loaded question, but a pretty easy answer, or so I think.
M4 rifles typically have what is known as a Government profile or an M4 profile to the barrel. At one point, the SOCOM profile, or heavy profile was introduced, with the main purpose being to hold an M203 grenade launcher better than the standard M4 profile. Both barrels are the same from the FSB to the muzzle, and share the characteristic slot whether the M203 strap would fit. Under the handguards, the M4 pattern is thin and comes up to 0.75" at the gas journal. The SOCOM barrel is thick under the handguards, and usually has vertical cuts, refrerred to as "M203 cuts" to support (better) the grenade launcher.
I hope I hit on the highlights. You cannot tell one from the other with handguards on.