In Vedic astrology, Mars, Saturn, Rahu, Ketu and Sun are considered cruel planets. Mars is the cruelest planet afflicting the birth horoscope of a person. If placed unfavorably, Mars is the most damaging planet for the married life.
Mars positioned in first, second, fourth, seventh, eighth or twelfth house in a Kundali creates Mangal Dosha. Mangal Dosha should be checked not only in Lagna chart but also in Moon chart and Venus chart. A person is Non-Manglik only if none of the three charts is afflicted by Mars. Mangal Dosha is also known as Kuja Dosha and Bhauma Dosha. The person having Mangal Dosha is known as Manglik.
North Indian astrologers don't consider second house while evaluating Mangal Dosha while South Indian astrologers don't consider first house while evaluating Mangal Dosha in a birth horoscope. However, there is consensus that both houses should be considered while evaluating Mangal Dosha and Drik Panchang considers first as well as second house while evaluating Mangal Dosha.
Many Kundalis have Mangal Dosha but in most of them Dosha is cancelled due to various combinations of Mars and other planets including state and house position of Mars and other planets in the horoscope.
If Mangal Dosha is present then all combinations which nullify it should be looked into. There are many rules which cancel Mangal Dosha. Drik Panchang considers the most widely accepted rules which cancel Mangal Dosha.