Is it possible to detect the mental flow state with EEG?
Bearing in mind the fact that I can't prove a negative, I'm going to say "No, it's not (yet) possible".

Flow is rather loosely defined (e.g. "merging of action and awareness"), so coming up with hard measures is a challenge in and of itself -- even without bringing electrophysiology into the equasion.  As a Positive Psychology concept, it belongs to the realm of "soft science" psychology, and this tends to make fuzzy things stay fuzzy.  (Please note that I'm not using "soft science" pejoratively).

I think the best you can do is measure correlates of alertness and attention (e.g. look for desynchronization in the alpha band).  If you come up with a very clever design, you might even be able to elicit ERP components related to conscious awareness of a stimulus of interest (e.g. P300b complex), but even that will be very difficult in non-laboratory conditions.

Alternatively, you could turn to machine learning to search for patterns associated with the conscious processing of a stimulus of interest.  If you want to go that route, I'd strongly encourage you to read one of my colleagues' Ph.D dissertation [1].  This dissertation proposes some novel approaches to pattern classification with EEG (and MEG/iEEG).  Note that this problem is a much easier one (classification of non-communicative patients).  That, along with the insane amount of noise you're likely to get and the need for a very inventive design, earns you a hearty "good luck" ;)




Characterizing the Electro-magnetic Signatures of Conscious Processing in Healthy and Impaired Human Brains