How to debug flow

If you set the debug log for user running the flow for Workflow to be Finer, it will show your assignment actions and values.

Here are the log actions you can get: https://www.salesforce.com/us/developer/docs/apexcode/Content/code_setting_debug_log_levels.htm

Perhaps the quickest way to see your sObject values is to add a Screen element where you display one or more fields. If you have a collection of records you'll need to loop through the collection, which means your screen will pop up each time, so if you have a large collection you'll want to try to filter it down for debugging purposes.

Another quick trick is to simply count the number of records in your collection to see if you're getting the number that you expect. Sometimes that can help you identify a problem before you inspect fields on each record.

'Hope this helps!