I would use the synthetic urine because it is always 100% effective if you get the temperature right. No they cannot tell if its synthetic. A donor urine sample would also be affective if you know this person is clean and you can obtain the sample shortly before you have to take the UA. It starts to go bad after 8 hours and I wouldn't push it past 6 just to be safe. They cannot tell if its male or female urine so your good there. You could pick up a thermometer at Walmart or Wallgreen's, I dont think GNC would have them.