Under most circumstances, it's obvious when a gas line breaks. You might hear a hissing noise or observe bubbling coming from the gas line. There will also be a very foul smell, similar to rotten eggs. If you hit a large gas line, you might even see the cement or pavement split and move.
All of the signs above tell you that it's time to call an emergency plumber. Breaking a gas line has serious consequences and could put you and your household in danger. You should stop what you're doing and leave your home. Open the windows and, whatever you do, do not use any electrical devices. If there's a small spark, it could lead to combustion. After a gas line break, your home isn't safe. You should work with a professional to test for the leak and repair the line.