How I can check if a string is likely to be generated by a bot?

I have spam issue. Some bot (I believe) is getting around Google recaptcha and inserting strings like the following into forms on my site:

dtbNPRpfcz

VvAJEXqueSKscY

Does anyone know of any JS or C# code I can use that would give a high probability of indicating that the above string is randomly generated?

If I could check the fields being filled and know that several of them were likely to be bot generated then I could block the submission.

The above strings seem to have more than a normal number of ucase chars for example.