You could use hydrogen sensors like the catalytic type. There is tapes that change color in the presence of hydrogen even in very small amounts.

Professor Yehia Khalil, Yale University, USA

Fellow of the University of Oxford, UK

By using hydrogen sensors.

Thanks, all of you. Any other suggestions are welcome. 

I want to find the easiest way to avoid the doubt of many researchers, who often don't know whether the evolved gases are H2 or something else during photocatalytic water splitting. Gas chromatography can be used later to quantify the gas evolved.

GC is the best not only to quantify but also to prove that H2 is formed

I strongly go with GS/GC though for time being or roughly if you want you can go with tapes, sensors,..

Electrochemical Hydrogen sensors can detect low levels of hydrogen. Minor modifications can also be done to these sensors.

http://pubs.acs.org/doi/abs/10.1021/cr800339k?journalCode=chreay