A photocell is I think the standard (or at least "the standard" in psychophysics applications I have also used). You need, though, to make sure your event cause

A photocell is I think the standard (or at least "the standard" in psychophysics applications I have also used). You need, though, to make sure your event causes a change that a simple photocell would detect (eg you have a corner of your screen or application window having a small square that turns from black to white and vice versa). You can connect that to an arduino or whatever you can use to get both signals in some low latency way (one from user interaction or whatever causes the event, and one from the event itself with that light intensity change). There are also photocells with a higher granularity for luminosity changes than just light/dark, so maybe you do not need to have the black/white square thing, just something that causes enough luminosity change to be detected. This should, in principle, give you very accurate timings if you can also get the event initiator somehow with low latency (or you can do stuff like simulate an event periodically and look for skipped frames/variance in the screen presentation rather than latency).