1. Drive down the road at 35 mph or higher and see if you get told off by the sign
2. Put a speed camera up and see if any vehicles are speeding but while the sign stays unlit.
If the camera actually counted vehicles/day and illuminations/day, then we'd have an interesting data source for traffic volumes and the percentage speeding- and you'd have an easy way to detect failure. But sadly, this is not a feature of these signs, which is a pity. It would then be of datamining value, rather than just decoration.
Now that the sign is apparently fixed, we need a test for that too. Again, the test is simple: drive down the road fast. In a five minute period on Saturday afternoon, 30% of the passing cars appeared to be participating in this test, which shows how much they care about signs working.
Here, for example, is the Audi A055GEY, showing that whatever speed it was doing, it was enough for the light to come on
A lull in traffic provides enough time for some pedestrians to cross. This crossing here is the main crossing point to any schoolkids going to the nursery school on the right-hand-side of the road (mostly from cars temporarily parked in the buses-only parking on the LHS), and for secondary school-age kids going to Cotham Grammar across the road.
With the school rebuilding going on, there is likely to be more pedestrian traffic here. Hopefully the 30 mph sign and a flashing 20mph sign on weekday mornings is enough to ensure safe crossings. That is if the sign works.
Again, a car volunteers to check the sign lights. It's not quite so obvious from the camera angle, but whatever speed it is calibrated too, it has been triggered by a Peugeot.
A period with the sign off would actually be a good basis for a "do these signs actually work" experiment. Turn it off for a month, measure traffic volume and average/peak speeds. Turn it on again, measure the same numbers. See if it has any effect whatsoever.
Correction: The first post called the councillor Neil Hamilton, who was in fact the Conservative-cash-in-an-envelope MP for Harrods; Neil Harrison is the LibDem councillor for Cotham. He says he determined the sign wasnt working it by watching it for 15 minutes as cars went past, and noticing that it never went on. Yes, that would work.