Even if you cannot see the moon in the night sky due to clouds or light pollution, you can still find out the lunar phase by checking a lunar calendar. Although the lunar cycle lasts roughly a month, it isn't exact and there is some drift over time.
The phase of the moon depends on the spatial relationship between it, Earth and the sun. The different phases are created by the moon's shadow, not the Earth's, as is commonly believed. The moon goes through a single rotation at the same interval that it takes to orbit Earth, the same side of the moon is always seen.
A full moon occurs when the 'daylight' side of the moon is pointed toward the sun. As the moon rotates, less of its daylight side is pointed at the sun, so it is seen from Earth as a waning crescent, then not visible, then a waxing crescent and back to full.
A lunar calendar includes both the time that the moon rises and sets each night and the phase of the moon at night. This can be indicated with a small image or text. Normal calendars can include information about the moon, or a special one can be found online.