Applications in Dock not showing 'open application' indicators Since up grading to High Sierra 10.13.6 I've had all sorts of issues, one of them is:

Being unable to tell which apps are open in the dock on my MacBook Pro. The black tags/arrow heads showing which apps are in use are gone!

How can you tell which Apps are open in the Dock now? Really really annoying problem and I can't see a fix anywhere.
System Preferences > Dock

Is ‘Show indicators for open applications’ checked?

Checking that should fix your problem with the Dock.