I prefer using the isAvailable expression function provided by @PGriffith 's module Ignition Extensions - Convenience utilities for advanced users
It's far better/faster/more efficient/prettier than using runScript.
There's also isGood provided in the standard igntion platform. isAvailable however checks specifically if the tag is both not disabled and exists. Whereas isGood only checks if it's Good, so will not be good if it has any config errors or other qualities you probably want to know about