you can see this article ''Colorimetric determination of resorcinol based on localized surface plasmon resonance of silver nanoparticles'' in this link:http://www.ncbi.nlm.nih.gov/pubmed/23016152 .Or you can use phthalic anhydride  on heating with resorcinol gives a dye fluorescein. This dye in NaOH solution gives a yellowish red solution with green fluorescence .then you can make quantitative analysis of your product by UV-VIS or FT-IR techniques .

Resorcinol reacts with hydroxymethylfurfural  (or furfural maybe) in conc HCl (4M is enought) to give a cherry red coloured adduct.  It should be fine at rt, otherwise try heating for a short time.  Can be monitored aroun 500 nm.  

Salvatori,

Please, look at

--- Prasad, M.R., Sugumaran, M., Vaidyanathan, C.S. A new colorimetric method for the estimation of resorcinol , Analytical Biochemistry 1977, 80 (2), pp. 483-489

Good luck