xliioper

You'd need to use ExpertFlyer to definitively check for Z since there can be Z bucket open with no Z fares for sale either because of advance purchase requirements and you are looking at flights that are too close-in to meet them, or because of dual-inventory checking and there is limited availability in lower coach classes (domestic Z fares use U/T/X/V fare basis codes and require inventory in them to book in addition to Z inventory).

But you can also use ITA Matrix with advanced mode options to search for availability for Z fares for sale (which will generally work for flights further out). Or you can just check on Z fare prices and then use Google Flights calendar view to check for availability based on pricing. For example, if there are Z fares available in both directions, pricing on ATL-LAX will be $1578 or less roundtrip. The 3-day roundtrip options below showing on Google Flights that are $1578 or less have Z fares available in both directions. Wed-Sat and Sat-Tue trips are generally pretty reliable for availability. But there are also some Thu-Sun and Fri-Mon options showing below.