First off, these questions should be asked for the major level ranges (feel free to modify slightly).
And on to the questions:
Can the class contribute meaningfully in:
1. Combat - This includes level appropriate:
a. defenses against physical and magical attacks
b. offensive capability; or
c. a way of enhancing party members such that they would realistically choose to have you accompany them
d. ability to engage with the threats in the environments you can expect to be fighting in for that level.
2. Investigations/information gathering
4. Diplomacy/social interactions with other denizens of the world.
5. Economic (crafting, professions, bartering, or other wealth-generating activities).
Feel free to debate or add other requirements you might use when evaluating a class. I've found using this type of checklist helps me avoid hyper-focusing on one area and entirely missing the fact that the class is going to end up sitting out large portions of the game in which they cannot meaningfully contribute.