Flowers on nepenthes grow from the apical bud - but since the flower is the terminus of the vine, in order to keep growing, the nep must activate a side bud. Even before you can see the flower, you'll be able to tell if your vine is about to flower if you see an activated side node. This may not apply to both, but it certainly seems to work with neps with large internodal spaces as you can see here in this N x Rokko and N x aristo x thorelli. I hope this information helps.
N x Rokko - Observe the activated side node:
N x Aristolochioides x Thorelli: