I have written several times, in various places, as have many others, that it is impossible to state whether a cartridge is .38 ACP or .38 SUPER with ammo that predates the .38 SUPER +P headstamp, without the original box label in hand. Ammo that is headstamped .38 SUPER will be that, unless it is a specialty load from someone like Dave Cumberland, formerly Old West Gun Room, that loaded some commercially-available .38 ACP ammunition for the older Colts, using the only brass he could get at the time, headstamped .38 Super +P. Even the pulling of bullets and weighing of powder charges offers no safe indication since many different powders, many not available to the average reloader, are used in factory-loaded cartridges.
The original separation by case material, plain brass for .38 ACP and nickeled brass for the .38 Super, went by the wayside in WWII when nickel was a critical material and eliminated from most commercial small-arms ammunition cases, and even before that in specific contracts for people who were not going to resell the ammunition and knew that despite a .38 ACP headstamp and a plain brass case, it was loaded to .38 Super velocities and pressures.
Of course, sometimes ammo such as this ends up being resold years later as surplus.
If you are contemplating shooting the older guns, don’t do it with any ammunition, regardless of headstamp, for which you do not have the original box label. Ofr course, the .38 Super, or most of them, works fine with .38 ACP ammunition - it simply gives “target pistol” performance rather than that of a full, self-defense load.