Are you serious? It should be obvious that your question is too VAGUE for anyone to provide a meaningful answer. We don't know whether you mean in an experimental/real life context, in which case the answer is, "you have to measure it somehow," OR whether you mean in the context of some theoretical problem, in which case the answer is, "it is either given to you already, or you are given enough information to solve for it using physical principles."
IF there is some specific problem that you require help with, then by all means post it using the TEMPLATE for homework help provided to you. Doing so will ensure that your post actually contains enough information for people to be able to help you.