It comes down to weave pattern in some cases as well as construction. Early WW2 sling generally used blackened brass fittings, later WW2 slings used a phosphate finished steel. The weave should be tight and linear, with no un-even appearance. Repro slings can have the extremes, they may appear to be very light in construction or conversely very heavy and uneven. Colors on repros are generally a sickly green or deep pea color. Originals will be consistently Olive green, depending on condition.
When looking web slings there are three basic WW2 variants;
The Kerr, "No Buckl" Sling, originally a WW1 sling, though was re-issued and saw service in WW2, blackened brass fittings.
The M1923 sling, a literal nightmare to assemble and use, and very short lived in service, had numerous blackened brass buckles.
The M-1 sling, very simple rig to use, phosphate finish fittings and an easy to use adjusting clamp.
The leather M1907 sling is the standby sling of WW1 and WW2 and is very easy to identify...two piece, one longer then the other, brass(WW1)or steel(WW2) double hook on the end of each piece, plus two leather loop keepers.