Hi
What type of app is it? Model or Canvas?
The answer however is the same. Use Monitor and have the user repro the issue, and in the Monitor it will tell you exactly which one.
1. Have the user close their App
2. Have them go to the Apps sectino of the Home Page of Power Apps
3. Find the app
4. Select the ... on the row
5. Click Monitor
6. Once monitor loads, click Play Published Version
7. Have the user do whatever it takes to repro the error
8. Go back to monitor
9. In the Filter (top right) type Error
10. Find the line wiht your error, there can be multiples, but only one of them will give us all the details we need
On the Line, that shows 4 Tables (in the pop out window), go to the 4th Tab
Open the Header And Open the Body Section
The Body and/or header will clearly tell you which Table/List
