It's a little extra work, I wouldn't call it a lot, especially when I create in TS2010 native mode and test in TS12 to ensure compatibility with both anyway. Quick and dirty way to know what's what is to find trainzoptions.txt in the root of 2010 or 12, open in notepad and add the line;
-ShowKuids
What that does is show the KUID number of whatever you have currently selected, yellow is built in, white is addon, red is not found on the DLS and is not built in.
That doesn't help for dependencies, quick and dirty way to test something like a loco is to select it in content manager, right click, view dependencies, view in main list. Select all and save to CDP on your desktop, then delete them all - built ins won't delete, whatever is on the DLS will show up with the DS icon, whatever is an unknown KUID you need to find out where it came from or change it for something else.
Worst trouble I had with 3rd party stuff was what came from where when - often a model builder releases a new loco from his website, most of the people downloading it have his other models already. Someone uploads a reskin and it has a bunch of unknown KUIDs, the guy who reskinned it never gave a thought to the dependencies since he's had them for so long he forgot where they came from. What I generally do with those is delete them and move on, unless it's something I have to have (like a Thomas the Tank Engine character for one of the grandkids, 20 years after my second divorce I somehow picked up another She Who Must Be Obeyed in Her Royal Highness my youngest granddaughter) in which case I open in content creator to find out what the unknown KUIDs are and replace them with something I do have.