It's however you decide to do it, and what your instrumental resources are. If you have a lot of pickers it's normal to go around in a circle, sometimes doing two or even three instrumental breaks between verses if there's not enough verses to space them out one at at time.

If it's a group of pickers unfamiliar with each other and the song, the leader can preface the start by saying "this is in G with a two chord (A) and the break is on the chorus" (or verse, depending on what you choose). If there's no time to say who should take the break, like "fiddle," or "banjo," or "whatever," then nod or look pleadingly with your eyes.

After awhile, in any group, you learn who you can depend on for a lead, or who's going to bring the whole shebang to a stop with a "deer in the headlights" look of panic and no music emanating from their instrument. (Happened just last night with a new bass player on "Mamma Don't Allow")

If you have a lead instrument who can do a short "turnaround" - the two to four bars that Mark mentioned - it's a good way to set the tempo and key, although it's always best to state the key at the outset.