There are some Aeration/Oxidation kits and electronic equipment available for testing sulfites in your wine. Below are some links to a few options.
Frankly, the first time you add sulfite is the easiest addition, since you know that, technically, you are starting with zero, so it's easy to calculate what to add. After you've added sulfite, it gets used up over time as it reacts with oxygen and performs its microbial functions, some also becomes "bound" in the wine. Testing allows you to determine your current sulfite levels, and therefore, how much you need to add.
Even though I have sulfite testing equipment (Vinmetrica), I only use it at bottling, and opt to maintain my sulfite levels during bulk storage by using the "rule of thumb" you've probably seen described here. That is, add 1/4 tsp of sulfite for every 6 gallons of wine as your initial treatment, and every three months thereafter until you bottle. I do test / adjust sulfite at bottling using my Vinmetrica.
During the first couple months of your wines life, after you've made your first sulfite addition, you may rack a couple of times, and that's OK, as long as you're not constantly opening the wine to smell and taste it. You can give it a small boost if you're concerned with multiple rackings. Once it's just sitting and you're leaving it alone, properly airlocked, you should have no worries at all with the 1/4 tsp / 6 gal method.
https://vinmetrica.com/wine-sulfite-analysis-so2-how-to-video/
https://morewinemaking.com/products/economy-aerationoxidation-free-so2-test-kit.html
