Most people don’t like uninvited guests in their house. If this is the case, don’t leave the door wide open for rodents and insects to enter.
When the sun is bright, take a stroll around the house. Pay attention to the foundation of the house. If there are any holes, take a piece of steel wool and stuff it inside. After a day or two, revisit the hole and if the steel wool has been dislodged than there is good chance you’ve got mice.
Mice are built in a peculiar way-their heads are softer than ours. What this means is that if there is even a penny sized opening they can squeeze through it. Insects need much less space obviously and so make sure to check closely. In particular, places where anything is attached to the house is suspect, including plumbing fixtures, like nozzles for hoses, or fuse boxes. If these items have even pen-sized holes exposed around them, the bugs will find a way in.
The best thing to do is to purchase exterior caulk and a caulk gun and then apply it all around any sort of fixture that isn’t fit tightly. Larger holes can be filled with cement and caulked afterwards. As for mice, know that they can climb very well, so if there are open gable vents in the attic, consider installing screen wire to keep them out.