What should i check if baby is constantly crying?

Many possible answer: Depending on the age of the baby, it could be colic, hunger, thirst, constipation, discomfort from too many clothes, irritation from the diaper, feelings of fear, loneliness etc. Illness is usually signaled by fever plus other signs, so if none of those, try holding the baby, talking to it, check for sweating from too warm clothes & correct; offer water, foods etc depending on age. Be attentive.