Generally, there are different ways of detecting transient spin-polarised paramagnetic species with EPR. Probably the most common is to use a conventional cw-EPR spectrometer in a direct detection mode – such as not to restrict the achievable time resolution – and recording transients.
In principle, it is possible to use pulsed EPR techniques as well to obtain spectra of transient polarised species.
Literature
Biskup, Till (2019): Structure-function relationship of organic semiconductors: Detailed insights from time-resolved EPR spectroscopy, Frontiers in Chemistry7:10
Forbes, Malcolm D.E.; Jarocha, Lauren E.; Sim, SooYeon; Tarasov, Valery F. (2013): Time-resolved electron paramagnetic resonance spectroscopy: History, technique, and application to supramolecular and macromolecular chemistry, Advances in Physical Organic Chemistry47:1-83
Weber, Stefan (2017): Transient EPR, eMagRes6:255-270