It's a bit tricky (easier in next release)

Anyway.

    tellmail status | grep Resync

Resync:   Tot=2038 0.0/s, Resent=0 Prune=0 Dir=14 Th=4

If the total isn't changing then it's finished.

(in new builds it will say finished reliably and give a progress indicator too)

    ChrisP.

How can I tell that the resync has completed?