Get current StartInterval status from Launchd

~/Library/LaunchDaemons isn't a valid location for either launch agents or launch daemons. You are misunderstanding the terminology. LaunchDaemons are system-wide processes, while LaunchAgents are run per-user. From the launchd man page:

~/Library/LaunchAgents         Per-user agents provided by the user.
/Library/LaunchAgents          Per-user agents provided by the administrator.
/Library/LaunchDaemons         System-wide daemons provided by the administrator.
/System/Library/LaunchAgents   Per-user agents provided by Mac OS X.
/System/Library/LaunchDaemons  System-wide daemons provided by Mac OS X.

Move your plist to ~/Library/LaunchAgents, and it should just work.

First, is this being opened by the system launchd (process ID 1, running as root), or a user launchd (running as a user)? If it's the system launchd, you're looking for a LaunchDaemon; if it's a user launchd, you're looking for a LaunchAgent.

Assuming it's the system launchd, you can look for the relevant LaunchDaemon like this:

grep 5901 {,/System}/Library/LaunchDaemons/*

(note: if port 5901 were listed in /etc/services, you'd also have to look for it by name as well as number. But it isn't, so you don't have to worry about that.)

It's also possible that it's a LaunchDaemon that was loaded from some other place; tracking that down would be messy, so start with the easy check first.