After finding the aphids, I sprayed all of the plants in the greenhouse just to be safe. Subsequent checks have not turned up a single aphid, but still I wanted a way to monitor for insect problems without peeking under every leaf.
My solution was: 3" x 5" Yellow Sensor Cards, also known as monitoring and trapping sticky traps. The traps are printed on heavy plastic and are water resistant. By attracting and trapping insects, it is apparent the extent and distribution of pests that need to be dealt with.
The traps have been in place for several hours, so far not a single insect is on either trap. So far, so good.
Over the years I have grown mostly tomatoes, cucumbers and peppers in the greenhouse. At no time have I ever had a insect problem with any of the above. Once, I added a single eggplant to the mix, and very soon it was covered with spider mites, just the eggplant, none of the others. It seems to me that some plants are just more attractive to certain insects than others, so since then I only grow eggplant outdoors. And, even more bizarre, when I have grown eggplant outdoors I have never found a spider mite on the plants.