But if you are using SP 2010 Client OM, it would actually throw an exception if the file doesn't exist:
using(var clientContext = new ClientContext(site))
{
Web web = clientContext.Web;
Microsoft.SharePoint.Client.File file = web.GetFileByServerRelativeUrl("/site/doclib/folder/filename.ext");
bool bExists = false;
try
{
clientContext.Load(file);
clientContext.ExecuteQuery(); //Raises exception if the file doesn't exist
bExists = file.Exists; //may not be needed - here for good measure
}
catch{ }
if (bExists )
{
.
.
}
}