If you don't already have one, go to radio shack or your grow shop and buy a scope-minimum 30x. I personally like using the 60x-100x with a little LED light-they are cheap, less then $30, but as a grower they are INVALUABLE!
You should be scoping for insects at least once a week, even if your plants look healthy. Again, google cannabis/pests/management for more info, but you should always be scooping your plants to catch any pests early.
Spider mites are one of the pests that are easy to identify. Google them and you'll see a ton of pictures/info about them.
They hang out on the undersides of leaves, so that's where they will be, check the leaves with damage first but also check multiple spots on your plant. If you have them bad enough you'll see them. Another way to check for pests is to get a white sheet of computer paper, hold it under your plant and shake it or tap the leaves-they will fall on the paper and you'll be able to see them.
If you find them, it's time to treat! Again, google is your friend.
Neem Oil and a product with Spinosad is my go to for spider mites. You want AT LEAST two different chemicals/compunds to battle insects. If you don't rotate products mites can build a resistence to one and you'll breed super mites that are extra hard to get rid of.
Since larva/eggs can hatch in as little as 3 days, and since the insecticides don't kill eggs, a treatment plant should look like this(and don't get lazy and only treat once or twice, you'll leave behind survivors who breed mites immune to the chemical you used)...
First, take the plants outside or somewhere else where you can work with them and not worry about spreading the mites.
If the infestation is REALLY bad I recommend removing the heavily infested leaves/branches. If it's really really really bad and it's a last ditch effort to save the plant, remove as much as you can(your not in this situation, just wanted to give you a detailed treatment plan in case you run into issues in the future).
After your done removing the worst of it, shake your plant to get as many off as you can.
Then it's time to treat:
I always start with neem oil. Go by what the bottle says, the brand I use calls for 1oz per gallon of water(I always use distilled or RO since i'll be spraying my plants and don't want any salts from my tap in there). ALSO put a tsp. of Dawn dish soap(the NON concentrated kind, I actually use Dr. Brohmers but that's hard to find) per gallon in there too, this acts as a 'wetting' agent and makes the spray more effective. I RECOMMEND GETTING AN ANGLED 1 OR 2 GAL. SPRAYER. it makes your life a lot easier, however a spray bottle will work too.
Mix your water, neem, and soap together. You want to do one plant at a time, starting from one side spray from the bottom of the container up through all the foliage(this is where the angled sprayer comes in handy, you can angle it up and get the under sides of the leaves soaking wet while the plant is resting on the floor, with a spray bottle you have to wear gloves and angle the entire plant), making sure you completely soak EVERY under side of each leaf. You want your plant dripping wet, make sure to try to cover every single part of your plant. Turn the container a 1/4 turn and repeat until your confident you covered everything. Don't forget to do the rim of your container and really soak the top of the soil.
HERE ARE THE TWO MOST IMPORTANT PARTS...
ALWAYS SPRAY RIGHT BEFORE LIGHTS OFF, AND RAISE YOUR LIGHTS so when they come back on they don't burn your plants-spraying with the neem oil is like putting baby oil on your skin and going out in the sun. Do it right before lights off and raise your light so when it comes back on you have time to check, see if they are dry, and ready to handle the light again.
AND
since this treatment only kills adults, and adult eggs can hatch in 3 days, ALWAYS TREAT EVERY 3 DAYS, AT LEAST 3 TIMES, THEN ONE WEEK AFTER THAT-Checking with your scope each day to see the results of your treatment(you'll see dead spider mites >: ) ). If you only treat once you won't get all of them and start breeding super mites that are immune to the insecticide.
Once mites aren't visibly active and your confident you stoped the infestation, continue to treat with neem oil once a week as a preventative.
Never use the same product for too long. Switch to a different chemical every once in a while so the mites don't develop a resistance.
Hope this helps.
I would get a scope and even if you don't have pests, I would start doing some preventative.