Sometimes, this is even broken out by type of music. Takes a bit of hunting, but open jams are occasionally listed in these publications. What's more, many of them hold open Blues jams themselves. Many music stores are staffed by players who are working a day gig while trying to further their musical careers. Call or stop in a store and ask the sales staff if they know if there are any jams happening in the area.
Chances are, they've got the inside skinny on where the jams are at.
Generally, a long shot, but it does work sometimes. The following is a listing of Blues Standards most commonly played at the Jams. Most of these standards have been covered by many of the great Blues artists over the past century. We list the Tune, the Artists who have recorded the song and the Writer. As you can see, we still need a little more work here.