I took a T-65, Imperial Defense "SA80", and a D&H teflon coated USGI mag and compared all 3.

T-65:
Floorplate tabs aren't cut to the proper size as the "SA80" or USGI mag. Can make Magpul followers a tight fit. Floorplates are blank.
Feed lips aren't the same either
0.010-0.020" narrower on the sides
Stamped various letters and numbers on the front of the bottom (e.g. "C" and "1")
Followers are black and tilt pretty badly.
Springs are very beefy, about the best part of the mag. With magpuls they can feed reliably.

"SA80":
Floorplate tabs aren't as clean-cut as USGI, but it is the right size, so Magpuls drop in w/o any problems.
Floorplates are marked
About the same size as the USGI mags (measured with 0.001" precision caliper)
stamped numbers on the front of the bottom (e.g. "3" and "4")
Black followers are molded with "ENGLAND" underneath.
Springs are very very wimpy, about 0.020" thinner than the T-65 and USGI mags.
Will fail to hold bolt open on last round sometimes even with Magpul followers.
You will need new springs and Magpuls to get the most out of these.

USGI / D&H:

Date stamped on the sides
Green follower still tilts, sometimes enough to prevent lula from locking down the cartridges to allow loading the next cartridge. Solution: Magpul followers
the rest you know...

Some things to note:
Imperial Defence and Royal Ordnance "SA80" mags are reported to be the one and same. Country of manufacture is supposedly Singapore and the English military has never seen these mags in service. True SA80 mags are the Radway Green (plastic floorplate) and the HK mags for the A2 of the L85/SA80. The Canadians are using polymer mags (Thermolds or Orlites?)
These mags are also reported to be made by Charter Industries of Singapore, now ST Kinetics.

T-65 mags:
I cannot confirm whether or not they are really from Taiwan. The quality is really cheesy and if it was made with Colt machinery (I cannot see why Colt would sell machines to Taiwan...) then the operator was a complete moron.

The Imperial Defence mags are definitely of a higher quality than the T-65 mags but the springs need to be replaced.

Another ARFCOM link

Go there for more info on the SAR80 mags.