Radiator tank or overflow tank leaks.

My overflow tank holds water until we travel a distance. When we come home after driving 100 miles, it is always dry. Sometimes, when I park the blazer there will be a small puddle of antifreeze underneath. It does not always leave a puddle every time I park.

My temperature gauge is always normal. If the overflow tank has a crack, how would I find it and patch it? Or would I have to replace it? Someone else suggested putting stop leak in the radiator. Will this stop the leak?
Any information you can give would be greatly appreciated. I don't have much money, because I am a single parent of 2 and I don't know much about fixing things, but I want to learn.

Thank you.