Check if file changed on SVN repo

I need to perform some operation if a target file is changed in the SVN repo. How can I test it? More specifically, I have a local version of a doc.tex file from which I generate a doc.pdf file that is not on the server. If my local version of doc.tex file is outdated I want to copy the doc.pdf file to doc-old.pdf, than update the doc.tex, generate the new doc.pdf file and, finally, compare the two pdfs. The only step I miss is the checking for updates of the doc.tex file on server.