If you are making a casserole at home, you can get a very good carb count by adding up the carbohydrates in the ingredients and then dividing the total carbohydrates by the number of servings.
Unfortunately, that technique doesn't work when you're not the cook. When you're eating at someone else's home or in a restaurant, use the following technique:
For a typical casserole with potatoes, rice or noodles, meat, vegetables, and sauce, you can figure that each 1/2 cup of casserole has about 15 grams of carbohydrate (equal to one fruit or bread exchange). The number may be off a little, but it will be a close estimate. Check your blood glucose two hours after eating the casserole. If your reading is higher than you expect, your casserole probably had more carbohydrate calories than the estimate. Next time you have the same meal, adjust by taking more insulin or eating less of the casserole.