I have had some success using letter pair frequencies as a language identifier for OCR'd text -- letter pair distribution is a much better metric than just letter distribution. Pick out all of the two-letter pairs in your string, and compare the ten most frequent pairs against some pairs you got from analyzing a training text ( just remember to take out the whitespace from the training text first ). If it exceeds some similarity threshold, it is 'real' text, otherwise gibberish.

For example, my own values for most common letter pairs in English are these:

English => ['he','th','in','er','an','ou'],
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I have found that a better than 50% match is a pretty reliable indicator

Be aware that the path you are going down quickly leads to AI-complete problems in natural language processing. This is another one of those programming tasks that seems very easy until you try to do it on real data.