I look for sign like runs going up the bank. Most of the sign though is a spot on the shore where the grass is chewed off or a mat of grass with chewed off grass or cattails. Like others have posted sign can be deceiving. I'll usually overkill an area and set on as much sign as I can. 4-5 days pull and move on. Thirty or more traps across 40 acres of creek is not uncommon, and that might yield only ten rats. More traps and less time. Ten sets might get the same amount of rats as thirty traps but will probably take twice as long. With only a few weeks before freeze-up don't have time to spend on one area.
Shame about den holes a 110 on a den will greatly increase your catch.