Encrypted data is (pseudo) random data, i.e. it has maximal entropy (near zero patterns).
A 140 character string which is e.g. English can be compressed to 108-110 characters with an off-the-shelf no-special compression algorithm. A wrongly decrypted 140 character string can be compressed to 138-140 characters.
(BTW the claim 108-110 is not randomly chosen, I actually tried this with 140 characters from a previous answer's paragraph. Longer texts will obviously compress significantly smaller than that, even. Text will usually be something around 1/4 of its original size, and other "useful, non-random data" such as executables or even images will have anywhere from 30-70% of their original size, depending on what it is.)
This makes it rather easy to identify the case where you have found the correct encryption key (actually finding it is a different matter, I invite you to try and brute force 128-bit key, let alone a 256-bit key -- good luck).
Now with an OTP the problem is that unlike with a block or stream cipher where there is some albeit obscure correlation, the transfer function could be (and should be, as the key should be random) totally random, basically anything.
Which means no more and no less than you can get every answer out of an OTP, provided that you fill in the corresponding key, and there's no way of knowing whether that's correct. Every permutation is equally likely, and no bit in the message is related to any other bit (in the message or the key).
Within limits one could allege the same about other encryption systems (since 2128 choices is a huge number, almost "infinite" for most purposes), but in practice it's not the case. The number of permutations is comparatively small (compared to, e.g. 21120), and the bits within the message are not independent of each other, or independent of other bits in the key. Mind you, flipping any bit in an OTP will flip one bit in one character. But flipping one bit in a block cipher will have a 50% chance to flip each and every bit. They're not independent!
Which means that despite there being a huge number of outputs, there is only a very small number (often just one) that actually make sense, and you can identify them by their entropy.