I was told by my friend (old school gardener) that if it's rotten down well and indeed a good manure it will have little red worms in it, when we've been for manure to our local stables, we dug deep and sure enough we've always found the little jewels.
I don't know whether is it significant to the age or whether it just signifies it's good ole muck!! It's worked it's magic on our onions 
If you have the muck worms in it and it's fairly dry you might want to try bagging up several bags of it and covering it over for six or seven months outside.
In that time the muck worms will have eaten almost everything that they'd normally eat in composting manures and have pooped the waste out in their " casts " . The resultant leavings will be like large dryish coffee grounds but as slightly larger granules.
Worm casts are one of the most outstanding forms of plant nutrients you can get and boy are they expensive.
( I have approx 22 cubic feet of them bagged up which are 2 years old this March ).