How do you know if a verb is from the second or the third group when it finishes by "-ir"

You usually know that a verb is in the second group when it finishes by "-ir". However, some verbs of the third group also end by "-ir". The differentiation is then possible when you look at their conjugation in the present: a second group verb always ends by "-issons", "-issez" and "-issent" at the first, second and third persons of plural whereas in the third group you will find other terminations. For instance, if you look at the plural terminations of the verb "finir" (=finish) they are "nous finissons", "vous finissez" and "ils finissent". If you look on the other hand at the verb "courir" (=run), the terminations are "nous courons", "vous courez" and "ils courent". Therefore, "finir" is in the second group whereas "courir" is in the third one.