Try it in a bathroom right after someone has taken a shower. Try it right next to a heating vent. You should be able to find different places and days when the dew point is very different. On a dry winter day it is possible that the dew point may be below freezing, and then this method of finding the dew point will not work because you can not make the water colder than its freezing point. However, it will still work in a humid bathroom.
Relative humidity is closely related to the dew point temperature.
To find the relative humidity you need to know the dew point temperature and the air temperature. Look on the graph below and draw a straight line up from the air temperature and a straight line to the left from the dew point temperature. The place where these two lines meet gives the relative humidity. If the lines don't meet on one of the relative humidity lines estimate the value by guessing how far the meeting point is from the two nearest lines.