If the wiring is plastic insulated and no earth, it may well be in good condition, if it is rubber and no earth, that rewire will be needed sooner rather than later, the rubber perishes, dries out and flakes off, leaving bare wires.
Unscrewing the odd switch plate will tell you. As no earth was common until post 1967, if it is towards the end of that period, and the PVC cable does not go though polystyrene (which attacks it), and has not been overheated (causes a green goo to exude from individual insulating cores of the wiring) then it may well have a few dacades of safe operation ahead of it.
It used to be common to borrow an earth from the nearest power circuit for the odd metal light fitting that needed it, and while this is still allowed, it is not really best practice, and if you are a believer in NICIEC rules then you have to run this cable in the same route as the original twin. IEE only require it to follow approved routes, and not be vulnerable to damage.
In the long run, save up for a re-wire that replaces all the old twin with twin and earth, but unless the cable is decaying fast, dont lose too much sleep over it tonight ! Get it checked up though if there is any doubt at all.
