Script in CLI to Validate Credentials for all interfaces

I think it depends on what credentials you want to test.  If you just want to check snmp you can run the following:

Device -all -testsnmp

This will show you a list of failed items and then a total SNMP count at the bottom:

Device SNMP test counts: Succeeded = 3, Failed = 5

From the FortiNAC CLI you could search the output.master log for "failed to connect".  Failed to connect will print anytime that FortiNAC is unable to connect to a network device via CLI credentials. 

grep -i "failed to connect" /bsc/logs/output.master | awk '{print $14}' | sort | uniq

If you mean SSH, you can test your password with ssh client from FNAC CLI.