It is expected that a system/network administrator will have to at least try using a technical solution to solve a problem he/she's facing at the work, but in the end, as a system/network admin, you're not supposed to fight with users. If your equipment simply isn't the correct technology to solve your problem, solve your problem by making it a company policy (obviously a policy is no good if you don't state consequences of failing to follow that policy) that they aren't supposed to RDP to their home servers (I suppose by RADMIN that's what you mean). If you've blocked the default port, and you know people are still doing it, then obviously you have some way of finding out.
Another way to look at it, it really shouldn't be that difficult of a task to find out what outgoing ports need to be opened. If you're really unsure, then co-ordinate with team-leads or head of departments (this should get you the information on 99% of what needs to be opened, and the rest can be opened/approved on a case-by-case basis)
An easy way I've learned to quickly figure things out is block all outgoing connections, allow those that you know are needed and wait for the phone to ring :) Or, another solution would be to allow outgoing what you already know you need, then at the end of the chain of rules, add a rule which will log anything else (since the connection didn't match any of permit rules, it will generate a log entry) and review the logs every so often during the day.
I also do know you can create a class-map and use regex to match information found within the traffic that goes back/forth, however I don't know enough about the RDP protocol (again, I'm assuming you're talking about RDP) to assure you this would work. I guess first and foremost the traffic would need not to be encrypted, and then you'd have to identify some kind of commonality in the connection negotiation traffic for a a session being established.
I've read your last post about the organization having 15000 users and such, and I do realize the answers I'm proposing are somewhat similar to the previous answer you got, but the truth is, as a business, what falls under "business related activities" should already be well defined to begin with. If it isn't, perhaps the problem is partly with the employees, but mostly with management for not making clear what's expected of their employees.
