Pretty tough question to answer without benefit of "vintage".
Generally speaking we think of barrel length and sight configuration and barrel taper, but there are so many exceptions that one needs to almost put a stake in "time" in order to answer.
For instance, the Masterpiece is generally thought to have a straight (non-taper) barrel, and that's generally true, but the very first models had a narrow ribbed barrel that did, in fact have a taper. Then, we think of the Combat Masterpiece as having a taper, but after the mid 90's those were deleted and became straight barrel.
Then, we
know that the M14 had the longer barrel versions, the M15 the shorter, but when the M14 was removed from production in the early 80's, the longer barrel lengths were added to the M15 line.
Both did have 5" versions, and they did differ due to taper/non-taper.
For some of us that have been around these guns for years, it seems simple, but the exceptions and production interuptions have made it tough to explain.
Here are some generalities, just to confuse things.
The M14 was the longer barrel model (6" and 8 3/8"), except for those special production runs that were made in 4"
and 2 1/2")
The M14 had the straight barrel, except for those early guns that had tapers.
The M14 had patridge front sights, except for those that had ramp (generally the 4" and 5" special production run guns).
The M15 was made in 2" and 4" barrel lengths, except those made with longer barrels...
The M15 had ramp front sights, except for the mid/late 80's guns made when the original M14 had a brief hiatus from production, those had patridge front sights.
The M14 generally had a wide rib barrel, except for the early guns that had a narrow rib.
The M15 had a narrow rib, except for those made in the mid/late 80's when the M14 was out of production for a bit, as well as those in the mid 90's when the taper was deleted and the gun had a wider rib.
The M15 was regularly offered in blue and nickel finishes.
The M14 was made in blue, except for a very few that were made with nickel finish (special production).
Oh, yeah, the mid/late 80's long barrel M15's had the same overall appearance to the M14's that they replaced, except for the length of the front sight base....
The barrel ribs on the M14 and M15 (and Masterpiece/Combat Masterpiece versions) had barrel rib serrations, almost always. The exceptions were the 4" Dayton M14's made in the late 60's, these had an extra heavy barrel from the M10 HB line, and the topstrap was matte, but not serrated.
When the M14 was shipped with magna grips, it had square bottom grips (exception was many of the Dayton guns
), while the magna equipped M15's had the modified Magna (PC) grips in later years (I'm sure there are exceptions for early guns as well!).
The M14 was offered in a SAO version, the M15 was not. There are examples of DAO M15's out there, but these would be special production if original, but I've not personally seen one documented.
Both the M14 and M15 was only offered in square butt gripframes, with the exception to the late 90's M15's and the full underlug M14's starting with the M14-6 engineering change and extending into the -7.
All this info taken into account, it's pretty easy to sort it out.