Short of crawling under the car to scrutinize the tranny ID plate, test drive it and get up to about 45mph, on a flattish bit of road so the OD engages. Once the OD's engaged, gently accelerate past 50. Just around 50mph, you'll notice a drop of a couple hundred rpms, like a gentle OD engagement - that's the lockup converter engaging. Or driving down a hill over 50mph, take your foot off the gas, if the tach doesn't drop a bit the converter is locked, and once you slow down below 50 you'll see the revs drop as it unlocks. If it doesn't do this, then it's not an 'L' tranny (or it's one that's not working...)
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina