You are standing in front of a screen you do not trust. Here is a browser, and here is what it can tell you.
Measured, not asked for. The refresh figure is what the browser is actually being given over about a second of frames — which is the honest number, because a display can report 60 and deliver 48.
These go full screen and are drawn at true device pixels, so the one-pixel checkerboard is genuinely one pixel. If it shimmers or turns grey, something between the browser and the glass is scaling.
Arrow keys cycle, Esc comes back. On a second screen,
drag this window there first.
This page checks the screen you are looking at. For what is happening on the cable — the panel's real resolution, or latency between two screens — use the test patterns in Deckboy itself.
For those you need something photographed, or something on the wire. The downloadable cards carry a frame counter and a latency clock for exactly that, and where latency hides is the long version.
Deckboy generates its patterns live at whatever raster and refresh the output is really running, rather than stretching a file at one — and a stretched test pattern tests the stretch. This page does that trick in a browser, for the ten minutes before doors when installing anything is not on the table.
Deckboy is a free, open-source cue player for live video. If the screen turns out to be fine and the problem is the playback, that is the part it is for.