Click on the (i) icon located in the top left of a report:

This will bring up the 'Specification' panel which shows you the make-up of the report - its metrics / dimensions / filters / dat rage / sorting / limiting options:
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You can also - if you have administrator privileges - preview the query / queries that are running to build the report, how long they took to run and when the data was last updated, (below).

The 'Filter ancestry' table lets you see how filters are feeding into your report, if they're being inherited from a folder or dashboard filter and any compatibility issues:

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