How to know if an SSID supports 4addr WDS mode?

I have a lot of nearby SSIDs broadcasted by neighbors. I intend to use them as backup connections due to the instability, bandwidth, and distance. I also have many APs around my house, and can use them as station devices and relay the connection to my main router for that purpose. Note that I don't want to use the APs as routers, but rather using them as station devices and let my main router do all heavy lifting work.

If the neighbor APs support WDS, I can simply enable WDS on my APs and bridge such wireless interface to a VLAN and push all back to my main router. If they don't support WDS, I also already know how to use relayd to relay traffic back to my main router.

My questions are: is there any tool or method to know if my neighbor SSIDs support WDS instead of actually trying it out? And if they don't, what will happen if my APs try to send 802.11 frames with From DS and To DS bits set to 1 (aka 4-address mode) to them? Are the frames dropped?

I am also appreciate if someone can explain the internal working of relayd. I know how to use it, but I still don't understand how it can relay traffic from 2 subnets using only 3 addresses in the 802.11 frame.

Yes, the beacons contain this information. WiFi analyzer app?

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