I don't know about a "standard" test. Jogging all the axes through a large range, exercising the robot fast for a 30-min or so period, checking the axes for looseness or leaks, checking the KSS revision and any/all installed Technology packs, are all things I would do.
Oh! Also, make certain that all of the install CDs and electrical prints are included. If not, insist on copies or don't buy it. CDs must include the special KUKA-specific version of Windows, the KSS install disk, and the CD for any software option packages.
Hm... test the CD-ROM drive. If it doesn't work, re-installing the system if something bad happens will be impossible. Test the floppy drive, too -- some KRC2s can't make/restore Archives to USB. Check that the USB ports work. Check that the default password (kuka) works for logging into Expert and Admin user groups.
As for what else... well, it depends on what you want to do with the robot. Older KRC2s can't run the latest versions of RSI, can't support the realtime Ethernet card, might not be able to run certain option packages. You don't want to try doing CAMRob milling with an older KRC2, for example.
Hm. Well, if the robot boots up properly to the KUKA HMI user interface, that's the first step. Then look for any error messages that won't clear. That's the closest to a POST test that you'll have without actually doing some hands-on tests.
Is KRC2-ed5 with 5.66 ok for realtime ethernet?
Robot KR210-2 serie 2000 from 2009 but the control box is from 2007.
That sounds like it should be one of the last-generation revisions of the KRC2, so probably, yes.
The best way to be 100% sure would be to take the controller serial number, contact KUKA Service with that number, and ask them if this controller can accept the realtime ethernet card for RSI. And while you're at it, ask how much it would cost -- it's likely to be quite expensive, sadly.
It's a standard (but very specific) ethernet card, so finding one on the open market shouldn't be hard. But the hard part will be getting the special KUKA-specific drivers that make it work with VxWorks (and make it invisible to Windows Plug&Play).
