They usually say the batch number on a sticker on the back of the bottle. Actually, I think the 500ml ones have it printed on the back label, and the 750ml have it as a sticker. Granted, some of the older releases do not have this.
For Atrial, Nocturn, and Aurelian (I think all the 500 fruits) the blend number should be clearly displayed on the label, where Andrew pointed it out. For other JK beers, the newer ones will have a clear sticker with the bottling date on it, like this You get into trouble with older bottles, as some didn't get labels. We drank a Batch 1 Funk Metal recently which had no date or label of any kind on it; the person who had it knew what it was, but otherwise we would have never known.
