Each time you go to weigh something on the scale, make sure nothing is bumping it and that it is sitting on a flat, hard surface, then turn it on or zero it. Next, if applicable (like when weighing a guinea pig), place your weighing vessel on the scale and rezero (tare) the scale; then get the piggy into the vessel and weigh the piggy with the vessel. If you're using the same weighing vessel again, you could just swap out what/who's in the vessel, but it may be easier to just start again from zeroing the scale if you're questioning the scale's accuracy. If you're not sure how your scale is zeroing properly, just turn it off and turn it back on. If you're using an analog scale, there won't be a zero function, so you'll have to do the math yourself to deduct the weight of a weighing vessel.
Make sure that your weighing vessel isn't causing you to exceed the weight that the scale is rated for ... many kitchen scales in the US are rated for 5lb or 10lb/11lb ... that's 2267g or 4535g/4989g, so for a larger guinea pig, you could potentially exceed the capacity of the 5lb scale with a heavier weighing vessel. I imagine scales sold in other countries are rated in kilograms, but I couldn't tell you what the normal would be, though I'm guessing similar. I use a lightweight plastic bin to weigh my girls, though cardboard boxes are also popular.
Here in the US, I looked back, and my girls that had lice were given Revolution, which is a brandname for selamectin, a drug in the same class as ivermectin. They were teeny tiny (around 6 or 7 weeks old), and it was done by weight, so their dosing likely wouldn't be relevant to most pigs, but it was not a full dose that would have been given to a puppy/kitten. I believe the vet said she used the same product for mites and lice. A fine toothed comb is good for removing debris from the fur. One of my gurls was long-furred, so she was getting combed anyways, I just used a finer comb; the other got combed just to remove debris.
If you think one guinea pig has them and the other doesn't, be careful when handling the two, as well, mites cannot live off of a host long, but you don't want to potentially transfer them on a sweatshirt/towel that you used when holding one then the other, for example.