Metzgeria furcata has a midrib which is clearly visible over the rest of the thallus (which is 1mm wide), which also extends to the tip of the thallus, although without an excurrent nerve. The defining characteristic is definitely the fact that the thalli and the midrib fork at the end, which is visible on most patches. The Forked Veilwort doesn't often make sporophytes, although on the underside of quite a few patches an inflorescence can be found.
Apometzgeria pubescens is very hairy and Metzgeria fruticulosa and M. temperata constantly have gemmae on the thalli. M. conjugata grows on rock, but M. furcata can sometimes grow on rock too, but M. conjugata has thalli that are twice as wide as M. furcata and the margins are much more turned down.