Just throwing it out there, but the automatic sub message is normal for damn near everyone on OF. When my ex and I used to post to my account, we had a welcome message to because we found that most people wouldn't message first (maybe they're shy), but after we added that welcome message, the interaction was near instant.
An easy way to tell if someone has an agency is them being online 24-7 and answering you near instantly anytime of the day. Plus, agencies are expensive so you're not going to find an unknown using an agency to run an account that likely won't get 100 subs. Lastly, I've never heard of agencies running free accounts because they still have to get paid and there's no guarantee that people will buy the PPVs, but paid subs are guaranteed.