Behavioural specifications allow to focus only on the"observable" behaviour of objects. These observations are made through "observable contexts" which are particular terms with a hole to be filled in with an object. We consider behavioural specifications based on the observation of a specified set of linear terms. The set of observable contexts is often infinite; therefore, we give an algorithm for computing some special contexts that we call "covering contexts", and show that they are sufficient for proving that two terms are behaviourally equal.