How do I test the signal strength of my apple router?

If you have a Mac, you can easily test the signal strength by using the Wi-Fi icon RSSI menu item.

To do so,

• Hold down the option key, and then, click on the Wi-Fi icon on the macOS menu bar.
• Note the value for RSSI. RSSI = Received Signal Strength Indication and is measured in dBm. It indicates the power level of the signal as it is perceived by your Mac's built-in Wi-Fi antenna. The higher (less negative) the number the stronger the signal. RSSI is measured in arbitrary units. Apple uses a scale of 0 to -100.

There is also the wireless diagnostics.. which has been available since Mountain Lion.

When you Option-Click the wifi icon you will also see the option to wireless diagnostics..

The diagnostics themselves are pretty much useless.. but there are other options like scan which are going to help show if you have interference issues.

how to correct if possible?

The only real way to correct is move the airport closer to the Mac.. or Mac closer to the airport.

Get another airport and use extend wireless.. over ethernet is far superior to over wireless.. but even the later can help in a poor area.

Get a better wireless router.. Airport have medium to poor range cf the best out there now.. even the latest airports were never competitive with the best in terms of range.. and the best kept getting better.. new models year on year.. airport range is now nearly 5 years old. No improvements.