The easiest test is by substitution with a known good backlight. I believe, but am not certain, that the dual USB's backlight is fully compatible with the clamshell's B/L. At minimum, it's electrically compatible, so for a test at least, it will suffice.

That said, in the absence of an electrical test, we can infer some odds: If the B/L was yellowish and dim prior to going out altogether, then the backlight itself is likely the problem. However, if it was bright and white before suddenly going out, then the inverter is probably the problem.