The way I checked some old ones was with an oxy and an ohmeter, the resistance will vary as you heat them up (but don't burn them!).
The only problem is, an oxy sensor can appear to work, give fluctuating voltage or resistance with all the above tests, and still be crook - giving innacurate readings to the ecu (they should generate a fault code). An engine analyser is the only true way to check it with a 4 wire oxy sensor up the exhaust for comparison and the diagnostics plugged in. If they are crook, replace them.