Summarize the following:
Turn on the tap nearest to the water heater and allow it to run for a full minute. Once it’s as hot as it can get, hold a drinking glass or similar container under the stream until you’ve caught a few inches. For the most accurate reading possible, it’s best to use a container that’s been kept at room temperature. Have your thermometer on standby so you can drop it in as soon as you’ve filled the container. Make sure the probe is fully submerged, then wait 30-60 seconds for it to gauge the temperature.  Write down the number you get for reference. It can help you work out the ideal temperature range for your home, or point to potential heating issues outside of the unit itself. If you don’t place the thermometer in the water right away, the water may have a chance to cool down enough to throw off your reading. Assuming that the temperature is at or around 120 °F (49 °C), you can rest assured that your water heater is performing up to snuff. Any lower than that and it may need to be bumped up a few degrees. Remember that temperatures exceeding 120 °F (49 °C) will be too intense for most home. Increase your water temperature in 10 degree increments to reduce the risk of scalding. It will take a little time for your water heater to catch up to the new temperature setting, so be patient while it reaches the desired warmth. In the meantime, hold off on running a bath or operating any appliances in case the water being circulated is hotter than you intended. Take care of any necessary adjustments before everyone in your home begins their daily routine.

Summary:
Fill a glass with hot water. Insert a cooking thermometer into the hot water. Determine whether the water is hot enough. Wait 3 hours before retesting the water temperature.