comparing market index to a potential stock

i've been exposed to the idea of the need to check e.g. the sector index or the main index such as the all ords or asx 200 b4 you do a trade. that when the main index is going down, it's not advisable to take a long position right. and we should check to make sure it's going up b4 we go long. let me know if i'm wrong. but i'm just not sure how i would practically do this ???? eg. for short term trades should i check that the all ords is also in a short term up trend , or what??