Hello, I'm Jason. I will do my best to answer your question quickly and accurately using the information you have provided.
Ok, there are a couple of ways of going about this. What I would strongly recommend is doing a proper key-off draw test.
However, the other thing you can do if you'd like to confirm a possible blower resistor fault is to shut the car off, get inside, lock the door, and listen very carefully for the blower motor to be running at slow speed. It's often very apparent
And with your observation of the fan speeds changing by themselves, this is a good bet
If you'd like me to walk you through the draw test procedure, I'm happy to do so. You will need two people, and a good multimeter that can display milliamps