Have used my hands, and legs/feet for locating runs. Have also used a whippy sapling (no branches) to "sweep" for runs in deeper/murky water. Put the tip end in the water, holding at an angle, apply a little pressure, and when it comes from shallower water to the deeper run, you will feel the tension release. Then its a simple matter of "defining" the run to know where to set. Coupled with a long stake and wedge, or Lefler style stablizer, has worked well on the deep runs for me.
Early morning scouting may also reveal the runs from the "mud path" in the run from their activity.
Depending on your state regs. (disturbing a den), you might also use a section of 1/4" rod to probe vertically around a hut. You will feel the difference in the bottom where the runs are~ the solid mass of grasses etc. feels different than mass then water (entrance). Then it's just a matter of locating the main run as it enters open water.
Another option for your 110's may be the "Elbow Set". It's geared more towards mink (Rich Faler, or Jim Spencer has it in his mink book, and had caught 'rats with it). I think if you made the top hole look like a muskrat dive hole (and the set near/on bank sign) you will pick up some extra 'rats (and mink especially if baited with a muskrat leg). Labor intensive, so I only have used a few (setting for mink/coon). Both are good books IMHO (and if you want more details on the set).
Another option is to set multiple 110's in the runs/trails, spaced a few feet apart. Learned that trick from riding with another trapper. That day he was 4 for 5 at one stop in about a 15' stretch. Most areas I trap are only good for 2-3 sets per run, but it has increased catch/stop (location) ratio, and reduced time in a particular area. It's a water version of gang setting~ simple and effective!
P.S.- I agree with Dale F on the competition. Lots of sign doesn't always mean lots of 'rats... just that they have been active.