One of the many useful options rsync offers is the ability to do a dry-run – in effect showing you what it would do without actually making any changes. To achive this we use the -n flag, like so:
rsync -n /source /destination
In practice we would likely use the -n switch in combination with other switches – commonly we use -avzP, so our real-world example would look like this:
rsync -avzP -n /source /destination
Thanks to the verbose flag (-v) you should see a full list of all of the files which would be transferred, as well as the final note on the last two lines:
sent 26457 bytes received 2086 bytes 19028.67 bytes/sec
total size is 26377806232 speedup is 924142.74 (DRY RUN)
…letting you know that it was a dry run and that no files were transferred.