How to tell if biological control agents are present

Root borers

Crack open the root and see if larva (white grubs) are present or if the root has been mined out.  There will be sawdust type material and the root will look fatter.

Seed feeders

Crack open the flower head and see if larva (white grubs) are present or if the head has been mined out.  On thistles you can look for brown marks on the back of the head 

Stem miners and defoliators

Check leaves for small round holes.  Look to see if the plant has been defoliated.  Check the stems for bore holes and brown marks.  The stem may look chewed on.