with the mites, unless its a leaf that shows the bite marks very clearly, you really need to move the leaf and look underneath. actually even if the bite marks show, they mites are pretty clever, they like to live in little turned under sections so you really need to flatten it out and get in there. ideally you'd look for eggs which are pretty damn small. many plants will also self-interfere (tomatoes), so you get little tangles of leaves that provide great hidden living spaces for them.