I appreciate both previous answers and they were helpful but not exactly what I was looking for. After considering the responses and doing more research I'm posting my solution so that maybe it will help someone else.

Button code:

 <asp:Button ID=btnSave OnClick=btnSaveClick runat=server Text=Save OnClientClick=return CheckForVersion() />

Javascript:

<script language=javascript>

    function CheckForVersion() {

        PageMethods.CheckForVersion(aspnetForm.ctl00$ContentPlaceHolder1$ddlPageName2.value, aspnetForm.ctl00$ContentPlaceHolder1$txtContentName2.value, OnSucceeded, OnFailed);

        return false;

    }



    function OnSucceeded(results) {

       if(results) {

            //version exists so prompt user

            if(confirm(Version already exists. Do you want to overwrite?)) {

                __doPostBack('ctl00$ContentPlaceHolder1$btnSave','');

            }

        }

        else

        {

            //version does not exist so save it without prompting user

            __doPostBack('ctl00$ContentPlaceHolder1$btnSave',''); 

        }



    }



    function OnFailed(error) {

        // handle pagemethod error

        alert(error.get_message());

    }



</script>

C# using Subsonic 2.1:

[WebMethod]

    public static bool CheckForVersion(string pageName, string versionName)

    {

        PageContentCollection pages = new PageContentCollection().Where(pageName, pageName).Where(versionName, versionName).Load();

        if (pages.Count > 0)

            return true;

        else

            return false;            

    }