A hard and fast way I can think of would be to make a call to the access control settings of an SSID that is configured for PSK, if you see the PSK in clear text you have a RW key if you don't you have an RO key
This is after a recent security change Meraki Engineering did.
If you don't know the owner, it isn't so easy as the keys are effectively personal info and anyone with a direct login can create a couple. I wish there were a better approach to key management, at least for MSPs.
As @PhilipDAth points out, you have to test each network to be sure a key isn't read-write on just a subset of the org.
One way to test the key could be to iterate through all networks doing...