If you know the stars, look at the constellation in the Zodiac. For example, according to astrology, now the sun is in Aquarius. In reality, it is one constellation backwards, so it is in Capricorn. Then, look at the starred sky - which constellations of the Zodiac can you still see? E.g. if you see (left to right), Leo, Cancer, Gemini, Taurus and horizon, then you know that there is still Aries, Pisces, Aquarius below and the sun is now "at the end" of Capricorn. So actually if you count cca 24/12 = 2 hours per Zodiac sign, then it is cca 6 hours after sunset. This is simple, logical, doesn't require moon, nor any "calibration".
But if you look at the starred sky every night, it is much easier not only to orientate yourself, but also to guess time. E.g. in winter the Orion dominates the evening sky. So if you see it leaning steeply to his right side and low above horizon, you plus-minus know where the west lies, and also it indicates a coming midnight. And if you see the Lion leaning his head just above horizon, you know that you really spend too much time in the pub - the new day will start soon. :) Of course this more or less is valid now, in winter - the situation changes over year, so you must regularly calibrate your star clock. :-)