So the house I just finished remodeling, and am now living in, has a small mouse problem. We have caught a couple of mice, and suspect there may be more. Catching the mice does little good, if more mice are just walking on in. I have walked around the entire house, and have found no voids > 1/4" in diameter. There were a few smaller voids (no larger than 1/8") around entry points for wiring/plumbing, which I filled with expanding foam. My question now is- how do I find where the mice are entering the house? Any tips?
Sprinkle flour or talcum powder on the floor. Mice will leave tracks in the powder, eventually revealing entry points or nests. You don't have to blanket the entire floor, just near the walls/borders of the rooms, and "as needed" until you have the evidence you seek. Keep in mind that their typical range is 12-20 feet from their nest.
Look for mouse droppings.
Use an ultraviolet light to find urine stains.
Use light. If you can make the room dark, look for sunlight shining through cracks from outside. Or, wait until dark and shine a light through possible cracks, either from inside or outside, and have an assistant on the other side alert you to any light shining through.
Watch for drafts. If you're in a cold climate such as Minnesota, wait until it is -20 degrees Fahrenheit, so that you can feel the cold air blasting through the mouse hole. (That's how I found an entry point recently. The cold air was flowing into a basement crevice such that I could see a spider web waving in the air. Turns out there was a crack where an addition had been put on the house. This was confirmed by shining a light as described above.)
I've found that mice can get in almost anywhere. I don't know how your house is setup but I have a tree which is relatively near my roof. I've had mice get up on the tree, climb up and into the roof vent. If I were you I'd just drop some money on a good exterminator (check the Better Business Bureau to find a reputable one) and have them get the house a good once over. As for checking on your own, I'd look for a trail of mouse droppings or urine sprays. Mice don't really have any sort of bladder control and just let go wherever they are and leave droppings everywhere so you might find some near the holes that they frequent.
DISCLAIMER: I only say drop the money on a professional because I REALLY have a strong aversion to mice and bugs, especially when they're in my house. I don't mess around when I see one, I go right for the phone and call a pro so my reaction (as well as my response here) is tempered by this aversion. I'm far from an expert on the various vermin that may get into a house, but that would be my recommendation since mice are pretty clever when it comes to squeezing into small spaces.