Your mapping.txt file and a Bug reporter.

ProGuard outputs the following files after it runs successfully:

• dump.txt
• mapping.txt
  
• seeds.txt
  
• usage.txt

For debugging, the mapping.txt file is the most needed.

mapping.txt
This file lists the mapping between the original and obfuscated class, method, and field names.

Your bug reporter : I use Bugsense : https://www.bugsense.com which has options for you to upload your mapping.txt file, that way if and when runtime errors are triggered, you receive the bug report in an explanatory and clear manner.

If you rely on Logcat, you would be needing to search through your mapping.txt file to find the appropriate classes being referenced by their obfuscated equivalent.

I hope I made sense. :)

Source:
ProGuard | Android Developers
Crash Reports and Operational Intelligence for Android, iOS, Windows Phone & Windows 8 apps