How do you find a baby monitor hidden in your house?

You would need to look for the RF signal that the baby monitor produces. This would be difficult without knowing what frequency the monitor is broadcasting on. There are some shortwave radiOS that can be used; scan the higher frequency bands until you get feedback, and at that point you have found its signal; then walk around until the feedback is loudest and you will be close to it.

A quicker way to look for it is simply to look at all the power outlets in your house and identify everything that is plugged in. The transmit end of a baby monitor is always using power, and will drain its batteries, if any, within a couple of days. If the baby monitor is still broadcasting after a few days, it must be plugged into the wall somewhere, and there are only a limited number of places where that can be.