Best way to track source of lead through to conversion point

Bit of an age old question, but does anyone track the original source of a lead through to conversion? I am thinking this has to be done via Google Analytics. We can't see from the original source fields in Marketo, whether someone has come from a PPC campaign, an organic search, etc. Is the only way to do this by using UTM parameters to actually track it back to an individual? Marketo doesn't go down to that finer detail. I'm sure someone must have tried to do something similar. I don't know much about the analytics connector with Marketo...