If I bleed some water from a radiator into an empty jam jar, stick an iron nail in, put the lid on and leave it and the nail doesn’t rust, does that mean the inhibitor level is ok?

Control nail in normal water to check it is iron.

It’s a bit of a rough and ready test but it’ll give you an idea. Wire wool shows up corrosion quicker. You can buy proper test kits but personally I wouldn’t bother with either them or a nail in a jar, if I was in any doubt about it I’d just sling another bottle of inhibitor in for £15 or so.

I’ve got softened water in there at the moment and notice that a new rad says not for softened water so thought I wonder what’s going on in there.

Using sentinel x100 which says ok for softened water.

Drained a bit off and it’s fairly orange, which doesn’t feel great but the nail test seems to be ‘passed’. Seems like needs a drain and refill but not something I want to do until it warms up a bit.