I think the definition of 'Common' Czech, as you are referring it in your question, is kind of tricky to explain. Lets say there isn't one clear definition of a 'common' czech. In my opinion it always depends on which part of the Czech Republic the native person comes from.
As you probably know the Czech Republic has its border with 4 different countries speaking 3 different languages. That's why it has a lot of dialects and it also can happen that a person from the west part of the Czech Rep. commonly uses some phrases which a person form the east part would never say.
'Where are you from?'
"Odkud jsi?" (Standard Czech)
I don't know which level of Czech currently are you, however my advice is - don't worry about it! Keep up with the standard Czech which you can find/hear in language books, TV, newspapers... and you are all good :)