As you note, calling may not give you the correct information ... unfortunately.
There are 2 more reliable ways to determine if the property is a resort:
1. Here's a list of all 380 (!!) resort properties. This does include MGM and SLH properties.
https://www.hyatt.com/promo/hyatt-resorts
2. It is identified as a resort property on the property's list of amenities.

Thanks for the very useful information! I especially never noticed that resort icon in the list of amenities.