a stream or low area that usually parallels the low tide line. The deeper the better; ankle deep, knee deep, or waist deep water.. The best have large potato size rocks and shells (like conch shells) and/or dark dense sand. A typical runnel is anywhere from 5' across to 30' across.
You are looking for the lowest point on the beach, a place of sinkers and quarters. An eroded area.
Sometimes the low spot is at the top of the beach where a cut has occurred. Sometimes its in the middle where a runnel or stream shows up. Other times it might be near the low tide line in the form of washouts.
If the beach is flat and you are only getting lightweight targets, you need to "run and gun." Move until you see a beach with some features. The beaches near me are sanded-in (flat with no features) and tomorrow, I will be hunting a beach 60 miles from where I live.