The very high reactivity of such radicals is responsible for their  short half-lifes (about 10−10 s for hydroxyl radical) which explains why their concentration is generally very low. Their direct determination is quite difficult for some matrices. For example the in vivo measurement of OH radical by electron paramagnetic resonance (EPR) fails because the presence of scavengers leads to an extremely low steady-state concentration. The simplest way to detect them is indirect, i.e. by using a probe which reacts very quickly with them. In function of the probe that is used, different detection techniques can be used.  Nevertheless such kind of determination usually estimates the total amount of produced radicals and not their concentration (I Gualandi, D Tonelli Talanta 115, 779-786, 2013; C. von Sonntag Free-Radical-Induced DNA Damage and Its RepairSpringer, Heildenberg (2006)).

I give you some additional information.

Determination of hydroxyl radical is ussualy performed with the use of a probe. The product of the reaction between the probe and the radical are determined by HPLC. Several probes are used. The most common is the salicylic acid but the accuracy of the measure is debated. This paper can interest you (Determination of hydroxyl radicals photochemically generated in surface waters under sunlight by high performance liquid chromatography with fluorescence detection, Anal. Methods, 2014, 6, 8193).

Superoxide radical can be determined also without the use of a probe. Some electrochemical sensors exploit the superoxide dismutase to reach a good selectivity. Also the probe can be used.

The determination of peroxyl radicals is quite difficult. I carefully searched some papers that described their determination for a different matrix (atmosphere) some years ago, but I found nothing.