I can't see your pictures, but I have had spider mites in the past...and I've had them bad.
Most often, you can see the spider mites. Simply flip the leaves in the area you are concerned with and watch to see if any of the tiny specks you see on the underside of the leaves are moving. You can also see the webbing better if you give the plant a light/fine spray misting. The water will stick to the webbing making them more pronounced.
I had the 'red' variety, so they were more visible than the gray or yellowish variety (2 spotted). The red mites appear to be specks about the size of grains of salt and actually appear black on the leaves. They spread to everything that they can reach. I had them on several plants and they even set up shop on my humidifier at one point. I let them spread because I was breeding predator mites at the same time but I released the predators too late and they simply never caught up until most of my plants were dead. It was more of an experiment than anything, so it was more fun to watch them eat each other than anything. The plant loss was minimal because the project was a small scale project, so it didn't matter all that much in the end.
There are many ways to get rid of them. The most common method to remove is a firm spray of water. This will typically knock them off the leaves but some bonsai plants might be too fragile for that - I'm not a bonsai guy, so I simply don't know. You can also use your fingers to lightly rub all leave surfaces - this will kill them.
There are many sprays out there on the market as well. I use an pyrethrum aerosol product called Doktor Doom by Hydrofarm. It just takes a light misting every couple weeks to keep them in check. You have to hit the bottom of the leaves, though, or you will be wasting your time and your spray.
And as noted above, you can buy predator mites to eat the spider mites. There are many problems with this. While the predator mites breed twice as fast as the regular mites, you have to catch them early enough in the process to give the predators time to catch up before the regular mites kill your plants. Once the predators run out of food, they start eating each other until the last one simply dies. They won't damage your plants, but there is a balance that has to be maintained much like any garden venture where the predators have to have food - and a continuous food supply to keep protecting, otherwise, they just die off and once they are gone, you get spider mites again.
One last note - you'll need a decent magnifying glass to see anything in detail (like mites eating mites). For the most part, you just get accustomed to what to look for and act accordingly. You WILL be able to see the specks move. You WILL see the webbing, most often at the leaf nodes. You CAN see the specks traveling along the webbing if there are enough of them. You WONT see a spider looking thing with legs and a row of fangs. If you do have access to a 10x or more lens, then you can see them in more detail. Contrarily, they will always just be specks on your plants that eventually kill them if left to run their course.
Two Spotted Spider Mite:
Red Spider Mite: