Manure!

You can tell by the look and smell really. If it looks more like compost and doesn't smell much, then it's OK.
If it still smells quite farmyardy it is probably a bit strong to put round your plants etc. In the summer a couple of months can rot it enough, when it's colder it can take a year. If it has loads of wood chips in it, it could take years to rot down.
The best sort of manure in my experience is what you get from rotting down straw soaked in horse urine. People on my site however are a bit obsessed with getting plenty of dung balls in their heaps.
It's a complicated question which requires much discussion every time we get a delivery.

Wow, I never would have guessed manure age could be so important! Thanks for all the info, I know far more now but will have plenty more questions for you!
Kaye