Like Chase Goldman, I use the wiggle back method to identify bottle caps. If you have the iron volume on, right when you get to the tip of the you will get the iron tone. Almost 100% of the time it’s either a bottle cap or a rusty steel washer. It works with both the hf coils and the X35 coils.
When doing the wiggle back, if you get the iron tone, turn 180 degrees and do it again. If you don’t get it again after turning 180 degrees, dig it because it may be a good target next to a piece of iron.