A girlfriend doesn't become a common-law wife, probably because - as incitatus points out - there is no such thing. If you are being accused of living together for say benefit purposes, then there are many legal and human right friendly ways they can prove this. Do you spend more than 50% of your time there, do you have mail going there, is your washing flapping about on the line regularly etc etc. Are any of the utility bills in your name, where does your employer say you live? Where is your car registered? All these details could indicate you are living together......sad I know but true.