I've only had mites once in my birds and it was immediately obvious that they had a problem. They started losing feathers (new feathers that they had just gotten because they all went through a hard molt a few months prior), stopped laying eggs, and started shaking their heads constantly. I guessed they had mites and treated them without visual verification. I don't recommend doing this usually, but I needed to do my annual de-worming on my birds anyways so I treated them with a product that kills both worms and external parasites. Their relief was impressively quick, and by the next day they were laying eggs again.
I used to use Sevin dust to treat my birds, but don't use it anymore. The easiest way to dust a skittish bird is to stuff them in a pillowcase filled with your dust up to their neck, and shake them around. Get them well dusted and remove bird from the sack. Take care to keep the dust out of their faces and out of your lungs. Re-treat in 7-10 days.
I used to treat my birds regularly for potential parasites, but I have a much more relaxed attitude toward it these days. I now only treat for external parasites when there are signs or symptoms of them. I still de-worm my birds, but instead of doing it twice a year I only do it once a year unless there are signs of something wrong.
I hope this helps. Good luck.