How to confirm whether a device was fried by higher voltage
I inadvertently hooked up a 15V/3.42A adapter to a device—an X99 (RK3399) Android TV box... Is there a way of confirming whether I indeed fried the thing (like hooking up multimeters, or something)?

Probably no.
But you could power it as intended and look at supply voltages inside derived from the power supply. If these are absent or wrong then the input regulators or converters are probably dead. This would be an indication that high Vin was the cause.
BUT it is extremely likely that this IS the cause - but no simple action will confirm it with certainty.