20/40 coexistance

This setting governs the router's use of 20 MHz and 40 Mhz wide Wi-Fi channels in the 2.4 GHz band.  Wi-Fi standards mandate that routers must use 20 MHz channels whenever neighboring Wi-Fi networks are detected or when clients request it.  When this setting is enabled, the router will comply with this requirement.

If this setting is disabled, then router may not necessarily drop down to 20 MHz.

It's very unneighborly to use 40 MHz channels because it consumes 2/3rd of the available spectrum, so you should leave this setting enabled.

I WANT to use only 20 mhz, but I thought if I uncheck this coexistence, then it will use only 20 mhz. So basicaly if this is checked, router will use 20 mhz mostly, and 40 mhz when air is clear, and if it is unchecked, then it will use only 40 mhz? So there is no way to tell the router to use ONLY 20 mhz, no matter what?

And is there any way I could check what frequency is on at the moment? I have iPhone, android and desktop pc.