I haven't heard of an O2 sensor test. Onlt thing I can think of is test resistance of all sensors hot and cold and compare results. I guess one or more will be bad. Take results from the 'good' side as correct. Maybe compare with a good known engine?
Good - Fast - Cheap: Pick any two.
E23 735i; E30 318i; E32 735iL; E38 735iL; R50 Cooper; R55 Cooper Clubman.
E21 318i; E32 735iL; E34 535i; E38 730iL; E53 4.4i