Even better, the LZW algorithms are known to fail catastrophically on data with few repeating patterns. Zlib can produce a 'compressed' file larger than the original file. This should occur in the cases that you describe in your post. As you say, programs tend to compress well, due to whitspace and repeated variable names. So you only have to check if the return from zlib is larger than the original file.

____________________
Jeremy
I didn't believe in evil until I dated it.