If you're on the Group Check In page and entries within the selected group have completed custom data fields, you can view these in a couple of ways.
The default view on Group Check In will have all custom data minimised like so:
You can then choose whether you wish to view just certain data for some individuals or whether you wish to reveal it for everyone in the selected Group.
View custom data for specific individuals:
Select the arrow to expand that individual entry and to view their custom data. You can do this for one or several selected individuals.
See the custom data field revealed below the person's name, email and pass types. The name of the field will appear on the left and their answer will appear on the right.
View custom data for all entries within the selected Group
Select the arrow on the header row to expand all entries in the selected Group.
Expanding all entries will reveal everyone's their custom field data.