i have a northern water snake and when i picked him up today he had patches of scabs on him thay where on his sides under side and under his jaw

It's usually pretty easy to tell when a animal is infected, you can gently lift flaps where the skin folds over naturally, and if it's infected, it'll usually be absolutely teeming with little red mites in those areas. Scabs can be from anything, fights with another animal, getting caught on something, anything.