You should be able to tell, depending on the subject, by just looking at both prints side by side. If you can;t see a difference, its not the camera.

Quote:
If a 16 x 20 print looks good from a Canon 30D "8.2 mp" How much better or bigger would a image look from a 10mp or 12.8 mp etc... Are there any test???

Very little. You are better off shooting the highest quality lenses before you worry about 8 vs 10 or 12 megapixels, unless those megapixels are substantially higher quality.

Rather than guess, I performed such a test. The short answer is pixel quality matters more than megapixels, unless the megapixel gap is large. And small focus errors matter even more. And careful processing (RAW vs JPEG). And lens and aperture and camera stability.

Really: what constitutes pixel quality when a pixel is just five numbers, the three RGB values (0 to 256) and the XY coordinates that locate the pixel within the image matrix?