Hm, behavior is a sign, but watching my buck gives me a sure clue. When he tries desperatly to get to one does cage, even climbing up where no rabbit is supposed to climb, sits there grunting and nosing with her through the wire, and the doe is interested too - good time to set a new record for breeding...
Last autumn after the sex change fairy paid a visit I had a healthy litter of 7 at about 5 months (Just in time for her to postpone freezer camp, I keep them 16-20 weeks).
The other time I noticed one of the fryer does doing that short, 2 second digging bursts on the lawn when coming out of the cage, although does don't do it that way, but bucks when mature, well, too late, they were with my breeding doe and I had a second unplaned litter before winter :x .
If the digging fits are related I don't know, mine sometimes dig tunnels several meters long, then from one day to the next fill up and hide the entrance and show no interesst in digging for several months. When I left the old, rented house this winter it was half a day's work with the shovel to find and fill up all hidden tunnels in the garden.