A good scan tool should be able to monitor the voltage on the O2 senors and put it on a graph. If they are reading perfectly, that will tell you they are good. But if they are not reading properly, that will not necessarily tell you they are bad, as other things can cause them to read improperly, (ie bad cats). Sometimes their physical condition is a dead giveaway once removed, (they can get all loaded up with carbon). But they also can deteriorate with mileage.

__________________
2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp