I was trying to figure out how to do this, there's no easy way but there is at least a way.
Login to your server, execute the following commands:
(Use sudo for docker if it gives a permission error. If you manually installed, well I'm sure you can figure it out then lmao)
docker ps
From the output of that find the name of the docker container running Postgres.
docker exec -it DOCKER_NAME_HERE bash
psql -U lemmy -h 127.0.0.1 -p 5432 -d lemmy
\c lemmy
SELECT name from person WHERE local='t';
You can also do name,display_name instead of just "name" in the comment above to see their custom display name.
Hope this helps someone.