If the trinket is on NO target, it glows.
If it is on your current target, it goes in low opacity.
If it is up on a different target than yours, it shows in red.
There is an option in the "Custom Option" tab to toggle the display of the name of the enemy on whom you've put the trinket on.
You can also change the "red coloring" in the "Conditions" tab of the Weakaura.