H2DCFDA would be a cheap way to start to answer this question. It is non-specific, so it detects several different types of ROS. It's fluorescence can be measured via plate readers, microscopy, or flow cytometry. However, it cannot be fixed and it will oxidize over time during your assay. It will also oxidize in the freezer, so you should use it ASAP.

If you need a fixable reagent that comes in multiple different colors, I would probably suggest CellROX from ThermoFisher. It's more expensive, but has a longer shelf-life at -20 degrees C.

First which ROS species are you interested in?

Superoxide itself is almost completely innocuous, see my papers entitled

Hemoglobin Is Not a Biological Fenton Reagent

and

Erythrocuprein, also Known as Superoxide Dismutase, Is a Hydroquinone Oxidase, and Imparts Resistance to Mitomycin C

where there is a quiet a long discussion of superoxide toxicity.

Superoxide can however, give rise to toxic species H2O2, hydroxyl radicals, peroxynitrite, hypochlorite etc. The aforementioned species can be assayed for (most are short-lived so you assay for a trapped adduct, multiple colorimetric assays are available for these species). You can also assay for the damage resulting from these species within the cells, such as lipid peroxidation, and oxidative damage to DNA (e.g., the formation of 8-oxoguanine) there are many kits available to assay for these species).

Article Identification of ROS using oxidized DCFDA and flow-cytometry

Try this for method and alternatives

best of luck