Revit MEPThis tool examines the mechanical systems that you create in a project to verify that each system is assigned to a user-defined system, and properly connected.
To verify the connections for duct systems in the model, click Analyze tab
Check Systems panel
Check Duct Systems.

Revit displays graphical warning markers and web lines for invalid duct systems in the current view. Warnings display when the following conditions are found:
Click a warning marker to display a related warning message. In the related warning message, click the arrow buttons as needed to scroll the list of warning messages. Click
Expand Warning Dialog to view the details of the warning message.
Click
Check Duct Systems again to turn off the graphical warning markers.
You can display disconnect markers for connectors that are not currently connected. To control the display of disconnect markers, click Analyze tab
Check Systems panel
Show Disconnects. In the Show Disconnects Options dialog, select one or more of the following:
In the following example, Duct disconnect markers were selected.
Click a warning marker to display a related warning message. Click
Expand Warning Dialog to view the details of the warning message.
Click
Show Disconnects and clear the selections to turn off the disconnect markers.