Evidence of the presence of adult beetles can be seen as notches eaten out of the edges of leaves, but it is the grubs that do the major damage. A plant that was looking very healthy will suddenly die. On inspection the plant pulls out of the ground very easily having had its roots eaten.
I find the best method of control in the tunnel is to sacrifice a few pots of their favourite plants e.g. strawberries, heucheras, echeverias and polyanthus. These act as a magnet and I check these pots periodically and deal with any inhabitants!!!! I also use nematodes if the infestation is major.