You might want to check out NetSpot
Site Survey Software. It’s a robust tool for visualizing WiFi signal strength in your home, and they offer a free version that should do just what you’re looking for. You can grab it from their website at https://www.netspotapp.com.
NetSpot works by letting you load a map of your floorplan, then walk around your house with your laptop or mobile device to plot out signal strength in different locations. It creates detailed heatmaps that highlight areas with strong and weak coverage. It’s pretty intuitive to use, so you shouldn’t have much trouble getting it set up.
Another good thing about NetSpot is it also identifies sources of interference and other issues affecting your WiFi signal. This way, you can not only see where your signal is weak but also get clues on why it’s happening and what you might do to fix it.
A few other options you might look into if NetSpot doesn’t suit your needs are WiFi Analyzer and WiFiman. WiFi Analyzer is more focused on Android devices, but it’s pretty good for getting a basic idea of your WiFi environment. WiFiman, on the other hand, is available for both Android and iOS and gives you a visual overview of networks around you.
Give NetSpot a try first, though. The free version is quite capable.