How to make sure the potting soil is free of problems

I'd be inclined to completely replace it as you don't know how long it has been in there and may be exhausted.

I sometimes make my own seed compost in small quantities by sterilizing soil from the garden in the microwave. I put it in a plastic back to stop it making a mess and cook it for a couple of minutes, that kills off any pests/bugs. Then mix it 50:50 with a shop bought MPC.