- channel balance vs volume (see nasty result for L30 just posted by Wolf)
- ground isolation (risk of ground loops)
Channel imbalance is tested, as Amir said but is only valid for THAT device. Could be worse, better, different for every pot. Smaller sized pots are usually worse. You can 'partly solve' the issue by having some gain settings on the amp.
Small sizes of devices (everyone wants small and cheap with top performance) has real world limits.
It is much easier to design a well performing (volpot) device when it is allowed to cost a lot more and the device can be a LOT bigger.
For the E30/L30 users the solution is extremely simple.
Set the volume control to a high position on the amp and adjust volume on the E30 or via the source.
Top level performance in small cheap devices !
ground isolation (very, very rare !) can simply be tested with an Ohm meter.
Injecting a specific current is a totally different ballgame. This needs specialized equipment and even when a device doesn't show much troubles on the test bench with specific 'test signals' one throws at it you can still be bothered by audible ground-loops in home situations.
These loop currents can take all kinds of paths and may actually manifest themselves in another device as the one tested.
Using proper grounded wall sockets, decent interlinks etc. should give less problems.
Trouble shooting groundloops and making cheap solutions for it generally isn't easy.
So yes, one can make a test for it but may not be conclusive because real world situations may differ a lot from home situations.