In terms of salting, a good start for a casserole going into a 3-quart (or 9x13) baking dish is about 1 1/2 teaspoons of salt. This advice comes from Faith Durand, the casserole queen herself. She should know since she tested over 200 casseroles for her new book!
To round out the seasonings, we aim for about two teaspoons of mixed herbs and spices. This depends a bit on the spices you're using. Go easier on potent spices like cumin and black pepper, but fresh herbs can be added more generously.
Our other trick is to give the casserole a taste. Mix together all the cooked ingredients along with the seasonings, taste a spoonful or two, and then go on to add any raw ingredients, like eggs. It won't taste exactly how it will when it comes out of the oven, but it will give you a good idea.
How do you usually like to season your casseroles?
Related: Food Science: Salting to Taste