I found all the submitted answers to hard to follow. Here's my answer. What I used to get the changelist to which a workspace is synced was:

1. From a command window, navigate to the root directory of your depot
2. Then run p4 changes -m1 @clientname

@clientname is the name of your workspace.

The Perforce docs say:

The p4 changes command, when invoked with @clientname as a revision specifier, shows the highest changelist associated with file revisions in your workspace. Typically this is the same as the last changelist that was submitted before you did a p4 sync.