I have an SVN repository that has supposedly just been upgraded from 1.4 to 1.6. The upgrade process seemed to go very very quickly considering the repository is very large (over 300G when dumped to files). I'm able to check out from the "upgraded" repository - is there an easy way I can verify which version of SVN it's being served as?
Unless you have ran svnadmin upgrade on each repository, upgrading the binaries for Subversion only help with functions relating to Apache and certain Subversion operations that don't need the repository to function.
To see if the repositories have been upgraded, the link in the comment above from Andy Smith will help in that. Basically, your looking the for the format file and you should see the number 4 there since you are using 1.6.x.
Now, word of advice. Sometimes all the good stuff as far as improvements to the repository cannot be accomplished with the svnadmin upgrade command. If you have the time and skill to do this, an svnadmin dump followed by an svnadmin load is the best way to ensure you get all the new features and enhancements to a repository after upgrading the Subversion binaries. This is especially true since you going from 1.4.x to 1.6.x since good things like sharding were introduced and could not be implemented through the svnadmin upgrade command.