First of all you still have the navigation performance scales displayed in your PFD of course which show very precisely where you are in your RNAV corridor. Secondly you should be able to check your ND every now and then which shows you how far of course you are in 0.01NM steps (and yes, you should be able to fly that exact manually). And of course you still have the dreaded magenta line. FD off does not mean you have no other data available which can be used to fly.