Hi Omer,

Crude but field-friendly methods were described by Dudley (2002) and Dominy (2004):

Dudley (2002) - doi: 10.1046/j.1360-0443.2002.00002.x

Dominy (2004) - doi: 10.1093/icb/44.4.295

More recently, Robert Dudley (UC-Berkeley) and Christina Campbell (Cal State Northridge) have been using portable GC to measure the ethanol contents of fruits on BCI. Here are links to their work:

http://www.stri.si.edu/english/about_stri/headline_news/news/article.php?id=1683

http://meeting.physanth.org/program/2015/session45/campbell-2015-dietary-ethanol-ingestion-by-free-ranging-spider-monkeys-ateles-geoffroyi-an-examination-of-the-drunken-monkey-hypothesis.html

Good luck! -nate

http://icb.oxfordjournals.org/content/44/4/295.full

http://onlinelibrary.wiley.com/doi/10.1046/j.1360-0443.2002.00002.x/abstract

You can  try  ATAGO  products.... there are very good portable equipment.  for instance 

Digital Hand-held "PEN" Ethyl Alcohol Refractometer PEN-Ethanol(V)..... which fruits are you interesting to test ???

Thank you for the answer.

I would like to look at wild fleshy fruits (species yet unknown).

If this fruits are citrus type  -  you can try  SenzyTec As well....

Unfortunately, they are not really similar to citrus fruits. 

I will look at the ATAGO device. I was not familiar with them.

Thanks a lot!