When I search on a custom field I get no hits

[Mark_Price]

You can use a legacy action set or setup a ton of rules in decision diamond. It’s highlighted here:

Checking for null values in decision diamonds

I need to check if a custom field (app2 surname) has been filled in to determine what email template to use I can’t see anyway to check for nulls or not nulls in the decision diamonds.

Since you’re using the API, you can just send an HTTP POST with the contactId and then use a PHP script to check the field + run an action set if empty.

FYI- when searching, there’s a ‘Data Exists’ search option under Misc Criteria BUT it only works with standard fields, so wouldn’t really help in this case…