When it comes to checking bearings for wear, I do it more by feel and sound.
If the crank rotates smoothly and quietly, and there is no radial play in them, then the bearings should be OK.

You can see enough of the R. bearing to tell if it's corroded badly enough to warrant replacment...

I would suggest replacing it anyway though, if it has a metal ball retainer. The older TT R. bearings (w/ the metal retainer ) would sometimes let go at higher r.p.m. so it's pretty much standard procedure to replace it w/ a new bearing w/ a plastic retainer.

Check out this thread for bearing supply sources...
http://www.rcuniverse.com/forum/m_10272775/tm.htm