A simpler way would be to do two captures of the same source at significantly different recording levels - if one recording is a higher volume than the other, they're being resampled (since gain cannot apply to a pure digital transfer).

Since you'd naturally expect to be able to adjust the gain, the default setting on your hardware will almost certainly be set to resample - sound cards with bit-matching (such as the Creative X-Fi mentioned above) have a specific option than needs to be checked to enable it.