If your migrating these to another fortigate, than just copy the encrypted hash out and re-use it.
fwiw;
On the ASA the PSK are not seen in the standard
show command. You need to use the syntax of
more system://running-config or more system://startup-config or the file named that holds the cfgs if you run multi-contexts
i.e
more disk://admin.cfg from system context.
On the fortigate and just like with juniper, I don' t think there' s a simple hack with the recover of any PSKs.
If you don' t know the PSK nor the lifetime of the PSK, your better off re-keying all items , and specially for remote-accesss VPNs. imho