If you open up the file trainzoptions.txt (or trainzclassicsoptions.txt for TC), and add the following line:
-showkuids
That will force Trainz to show the kuid number for the item at the bottom of the screen. Any kuid with -# (e.g. -1, -3, etc) is an Auran made asset, or is built-in. If you are unsure about an item, write the kuid down, and do a DLS search for it. If it turns up on the DLS, you can be fairly sure it's not payware

If your using TRS2006 or newer, do a search in CMP for the kuid. If it's built-in, then again you can be fairly sure it's not payware (although this isn't a certainty, if it's built-in in that version, then you'll be fairly safe using it).
But, from memory, the 'OZ' textures are freeware built-in from the very early versions of Trainz, with OZ meaning Australia in many cases

Zec