An ESP32 or ESP8266 Site-Survey-Tool, code is provided for both.
Using an ESP32 or ESP8266 in Station Mode enables received Wi-Fi signals to be displayed with their SSID and Received Signal Strength Indicator on an OLED display.
RSSI units are relative and determined by manufacturers, so they cannot (usually) be referenced to a standard, but an RSSI of -40dBm is stronger than an RSSI of -50dBm.
You can determine the maximum likely received signal strength by placing the site survey tool next to your Router, where you will most likely get a reading of -40dBm, next move around your property and determine where the weak signal areas are.
Generally, a strong usable signal is from -40dBm to -70dBm.
Any signal less than -70dBM (e.g. -71dBm) will lead to unreliable connections, packet loss, then retries and poor link speeds.