Look at the verb which follows them. - after modals the verb is in root form . We could play gin rummy. We are playing gin rummy 2. Look at the meaning. In order to identify modal verbs in a language sample, we also must consider their relationship to catenatives.
Modals are similar to catenative verbs in that they express the same ideas.
Each of these sentences expresses the notion of necessity. However, the syntactic characteristics of catenatives and modals are very different. Here is a simple rule for distinguishing modals from catenatives in a language sample. All catenatives contain the infinitive particle TO, while modals do not. Yes if the utterance contains a modal, select No if it does not.