Before setting up an external trust with another AD forest, here are some steps that you can do:

1. Verify that there is a two-way external trust between the two forests.
2. Verify that the trust is established from the trusted domain, not the trusting domain.
3. Verify that the trust between the two forests has been set up correctly and is setup with the appropriate permissions.
4. Check to see if the DNS records have been set up properly for the two forests to communicate.
5. Verify that all DNS records are up to date and that the necessary hosts are listed with their IP address.
6. Ensure that the two forests are both in the same domain.
7. Verify that both forests have the necessary account authentication enabled.