Read the hypothesis statement carefully. Check if it compares two groups on the same variable. If the statement claims that there is no difference between these two groups with respect to the variable, it is a null hypothesis. For example, if a medicinal chemist claims that there is no difference between the antibiotic effect of drug A and drug B, the statement is a null hypothesis.
Find out if the experiment involves studying the effect of varying a parameter on the experimental outcome. See if the hypothesis describes the relation between the two. If it states that there is no relationship between the parameter and the outcome, it is a null hypothesis. For example, if a research study states: "There is no relationship between the amount of time that teenagers spend browsing the Internet and their academic performance," it is a null hypothesis.
Review the hypothesis to see if it speaks of a relationship between two variables that the study compares. Check if it predicts the definite nature of the relationship or simply states that a relation exists. In both these cases, the hypothesis is of an alternative type. For example, the statement "There is a difference in the antibiotic effect of drug A when compared with drug B" is an alternative hypothesis of the non-directional type because it specifies a relation, but not its direction. If the hypothesis states: "The antibiotic effect of drug B is greater than that of drug A," it is a directional type of alternative hypothesis.