i have said it before i know... clean out the grow space before you put plants in it. this is about the time they will show up though. i spray neem every week. mix some up with water and add to the soil around plants and it will get into the plant. bugs don't like the smell or taste so it helps keep them away. by the time you can see mites and aphids, they already have produced several litters of baby mites so don't wait to treat your plants.
get a microscope. if you look at a leaf under mscope you can see the damage, holes all over and you can also see eggs. they are there damaging your plants before you can see them. i have found two bugs, very tiny but they were both dead.