The biggest difference is the tang on the barrel, properly called the barrel extension.
The LW20 has a short tang, like an 870.
The LT20 has a long tang, like its big brother the 12ga 1100.
The easiest external visual clue is the shape of the ejection port. The LW20 looks like an 870, with the top and bottom of the port being parallel. The LT20 has a dogleg on the upper rear of the port, being wider in back so the rim will clear.
LW20 barrels are getting scarce. It is possible for a competent gunsmith to machine down the tang on an LT20 barrel to fit an LW20 receiver. Gander Mountain used to do this for $50, but I don't know if they still do.