Couple ways I have located them.
1. When the water is low in the late fall and winter, you can see clam beds along the shore. Fish them when the water is up in the spring. Also, clam beds will extend from the shore area out into the main channel from those spots.
2. Jigging along the main channel and either snag onto a shell or have a live one clampp down on your jig. If there is one, there is a very good chance there are many more in that area. Make a mental note of the location.
Basically, time on the water will give up the locations…..
-J.