Go to your report type (Setup --> Report Type) and look at the object relationships
- This will display the related list label. This is mentioned in the Add Child Objects to your Custom Report Type help doc.
NOTE The name of the primary object is derived from the plural label
field. The names of any related objects are derived from either the
related list label field or the custom field that defines its
relationship to the primary object.
You can cross-reference it matches with your expected lookup by going to the custom field itself. If you don't have unique related list labels, you can quickly edit this and it'll reflect in the report type with a refresh (and then change it back).
- Assuming it's new, you can also see similar info by clicking into
Edit Layout within Fields Available for Reports. If you look at the fields from your source object, you should see the lookup being used as the only field not already selected to display (highlighted in gold).
- As a non-UI option, you can also take a look at the ReportType metadata in VS Code.
sfdx force:source:retrieve -m ReportType
Instead of the related list label, it'll display the relationship name that you can cross-check with the lookup field.
<relationship>Accounts__r</relationship> //here you can cross-reference