I have no idea if this test will work on the Escort but it does on a Senator
24v. Generally known to my car club (Autobahnstormers) as the wah wah test. Goes something like this. Heat the engine up on a good run. With the car stationary hold the revs, with your foot on the clutch, at around 2k rpm. Now hold your foot completely still, the rpm should be rock solid. If the revs go up and down slowly (which will make a wah wah noise from the exhaust) the lambda is shot.I have seen lambda sensors die in a couple of ways, one of which is to "stick" at an arbitrary output voltage, easy to spot with a DVM. The other is a slowing down of the reaction time (which the above test checks for); basically I think the time-constant between the ECU adjusting the mixture and the lambda reporting it changes putting the closed loop out of sync and causing the effect!
Will P.S. Be careful removing the old one if it's been in the exhaust since 1994. I have just had a bit of a nightmare with the wife's Vectra, the front lambda sensor just unscrewed, the rear snapped in the boss, resulting in me having to remove the down pipe, shearing the bolts holding the cat on at the same time; I had to retap the threads; 30 minute job turned into 2 hours!