How do you diagnose if the water pump has gone or going?

2005 ST.

thanks

A coolant leak coming from the belt side of the engine, unsteady temperatures, a rattle?

Mine was a coolant leak, on the other side though my temperature issues (never getting to the half-way point on the temperature gauge, temperature dropping when driving) was down to the thermostat, and my rattle was the idler pulley.

my heating is rubbish, on hot its warm at best. but doing 60 and it seems fine, slow down and it shocking.
Dont know whether its thermostat or water pump

my heating is not as hot either as it was and I think my temp gauge is a tiny fraction lower than normal not much, I have seen it rise above that, suspect I need a new thermostat, water level all good, not sure if my IE4 mountune inlet has to come off to get to it ?
Sometime I'll keep car running and see if it gets up and fan comes on, unless it gets worse I'll leave it for now.

Thermostat done. £21. Heating is intense now lol

Think £21 is probably a record, was that aftermarket or did you just change the thermostat without the housing?

It was a Circoli one. Compete unit. £10 for the clip remover.