I'm not aware of a secure test to know if a sound card could capture bit-perfect; you can try to capture a laserdisc AC3 or DTS soundtrack - just two minutes - and play the result file through your digital amplifier; if it is bit-perfect, you will hear the right soundtrack; if not, you will hear noise - sometimes with wrong settings you could hear the right soundtrack for some dozen seconds, then only noise...
Probably the problem is not in the digital capture, but into the analog one... you could always use RightMark Audio Analyzer to test your soundcard: just connect the digital out to the digital in, run the test, and you should obtain something like this (or better):
Testing chain: External loopback (line-out - line-in)Sampling mode: 16-bit, 48 kHz
| Frequency response (from 40 Hz to 15 kHz), dB: | +0.00, -0.00 | Excellent |
| Noise level, dB (A): | -95.1 | Excellent |
| Dynamic range, dB (A): | 95.1 | Excellent |
| THD, %: | 0.0005 | Excellent |
| IMD + Noise, %: | 0.0052 | Excellent |
| Stereo crosstalk, dB: | -96.0 | Excellent |
| IMD at 10 kHz, %: | 0.0056 | Excellent |