A variant of finding out whether your worm casting is mature is the cress test: if the cress germinates and grows, then your worm castings are safe, even for sensitive young plants. Mature compost should only smell like soil and be crumbly.
Here's how you do it:
Sow a little watercress in a bowl with the test worm castings, water it and stretch a fish film over it.
Location: bright and warm, no direct sun.
If the cress germinates after 3-4 days and forms green leaves, the compost is mature. You can now remove the foil and watch the cress grow until you clean it up.
If the cress does not germinate or the leaves turn yellow soon, the compost is not yet mature enough.
