Lol thing is you can't really tell the difference...cos they are more alike then they are unalike...
for example they both mean and, they both mean then, they both mean so,
but faa represents order (tarteeb), like dhahabtu ilal suuq fa almasjid, i went to the market then the masjid,
and faa also shows something that occured but with no time between the events (ta3geeb) like jaa'a muhammadun fa zayd, muhammed came and zayd (directly one after the other)
faa (sababiyyah) also shows the reason for something... kammaltu assalaah fa raja3tu ilal bayt- i finished the salaah so i returned home, so the reason for going home was because the salaah finished
hope thats helped a bit inshaAllah...
its something i always get mixed up with too ukhayy,
but what you could do is, take the ayaah you mistake faa for waaw/vice versa and think of what it means in this ayaah, does it mean and so or then and then put that into memory...that helps
wassalaam