HANDLENS-worth every penny!
Anybody interested in IDing very small insects before they are a huge problem on your tomatoes should consider getting a handlens. Yeah, there may be people who have grown tomatoes for 30 years and never had one but today the buzzword is IPM, integrated pest management. You monitor carefully to catch problems before they are an in-your-face problem. Plus , they are alot of fun. I have seen a few miniscule specks on a leaf and then determined they were spider mites even before they created webbing. I have seen whitefly and all there life stages in an area less than the size of a dime and before they spread all over the place. You do learn with the handlens what do look for without the lens , if that makes any sense but I still like to use the lens to verify. They are a little tricky to get used to--hold it straight on your eye and manipulate the leaf till you see insect.
I have taken several entomology classes so I know what is recommended--get a 10X lens. Bigger doesn't help and is harder to see through. I would stick to a doublet or triplet 10X (see link below).
Sina 10x douplet 15.00
Economy 10X triplet $22
B&L 10X triplet $33
I got mine at a university bookstore but I found this source on the internet. It is so interesting to see bugs magnified you may want microscope next so be forewarned.