If you have the key, presumably you know the owner, so as above you can use https://developer.cisco.com/meraki/api-v1/get-organization-admins/
to find permissions.
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...
https://developer.cisco.com/meraki/api-v1/get-network/
...then try to write back the same info with...
https://developer.cisco.com/meraki/api-v1/update-network/
...I think this would work with both SM and device networks, even if they have no devices.