Depending of the type of report, there can be numbers of areas it can be tested.
Few of them are like:

1) Report layout (Attributes/Metrics positions)
2) Naming conventions
3) Totals & Grand Totals
4) Validate the Sql query
5) Drilling options - Simple & Advanced
6) Prompts If any
7) Prompt ordering, Prompt Defaults (If any)
8) Metric Calculations
9) Drill Maps (Inc. Templates)
10) Security filters (If any)
11) Export/Print functionality
12) Formatting Properties like alignment, scroll bar, decimal places, etc

Hope this helps :-)