How do I check a custom field has a value?
To my knowledge the current answer to this question is: you can't do that yet with the most recent D8 version of Rules. Refer to #d8rules status update November 2017 to get a better understanding of the current status of Rules for D8. Here is a quote from it:


  ... Fago can’t dedicate time required to develop the module further. What Rules is really missing at the moment is development capacity to help finish the last milestone.


One never realizes what has been done, only what remains to be done ... So start here if you wonder how to contribute.