I use NLog with next configuration:
This did the trick for me:
var fileTarget = (FileTarget) LogManager.Configuration.FindTargetByName("file");
// Need to set timestamp here if filename uses date.
// For example - filename="${basedir}/logs/${shortdate}/trace.log"
var logEventInfo = new LogEventInfo {TimeStamp = DateTime.Now};
string fileName = fileTarget.FileName.Render(logEventInfo);
if (!File.Exists(fileName))
throw new Exception("Log file does not exist.");