OK, I wanted to try to find out if there were encrypted data at some offset in a chunk of random data, I'd start with Knuth's tests for randomness. I'd break the thing up into decent sized chunks (1 meg or so) and run a bunch of different randomness tests on each chunk and on the whole data set and see if any patterns emerge.
The thing is, even if the encrypted data looks pretty random, it's likely to look DIFFERENT than the surrounding random data.
The worse problem is that if you have someone who's asking you if there is encrypted data, and they find some bogus pattern in the random noise, then you've got a problem because you can't prove that there ISN'T any data there. If you are being prosecuted in a normal US court, you might get away with this (if they can't prove that you've got anything encrypted, it may be hard to hold you in contempt trying to get you to give up the keys), but if you fall under the sway of some intelligence agency that doesn't like the look of you, it's not likely that they'll just let you go because you claim there isn't any data.