Critiquing any art form is essential to understanding the art itself, and it helps in forming your opinions.
Look at the actors and form opinions about whether you believe the character. Also determine if they are genuinely portraying the character in the right era and setting for the play. This is especially important for period pieces. Pay attention to their mannerisms and determine if they are set in the right era and not too modern.
Listen to inflection. Many actors will have the natural ability to emphasize and inflect the right words in their lines. When you critique acting, listen to those that are seemingly natural, and then turn your attention to those who do not seem genuine. Try to notice what it is they are doing right and what they are doing wrong. This can be anything from volume to their level of intensity throughout the show.