You can use the following command, together with a grep, on the server:
svnlook tree --full-paths <repository path> | grep <file>
That s the fastest option if you need to look into a big repository.
If you only have access to the client side, again with a grep:
svn list -R <URL> | grep <file>
should do what you need.
Both will recurse through the directories, with the client you may start elsewhere than the root of the repository though, but it will be somewhat slower as the information has to go through the network (possibly).
Edit: Also, both gives you the option of looking into a past revision (should the file be deleted):
svn list -R -r <revision/revision range> <URL>
and
svnlook tree -r <revision> <path>
.
Simply type svn help list
or svnlook help tree
for further details.