I guess it will depend on the cells you are working on. But here is an interesting paper to start with, you can find the whole methodology to confirm intracellular ROS production by flow cytometry.
A simple way to analyze ROS is fluorometric assay by a reader which can read both green and red colors. It is easy to measure both total ROS and mitochondrial ROS. Homogenous monolayer adherent cells are also good for this assay. Alternatively, one can measure ROS from ultrasonicated lysates of non-adherent cells.