What do I do if there is a situation such as an MCC for instance if the client has no record of motors being fed, and no willingness to look into it. My thoughts are to go worst case and load the MCC to its max capability with Motors but not sure if there is another way.

I think that is a bit aggressive. MCC's are typcially dominated by relatively small induction motors. Their contribution is going to be small and short lived. I would estimate the lineup based upon what the other lineups look like or what type of field equipment I see. Its okay to just pick a typical example and multiple, ie, not evaluate every motor, just say there are 50 each @ 10HP and 20 each @40HP. Estimates like this tend to be more accurate than you would first think, because the contribution of any one motor is nearly negligble when compared to the total. Synchronous motors need to be evaluated individually, but you don't see them much anymore.

For motors that I don't know I tend to approximate them based on the breaker size, using a common motor HP that uses ~80% of the rated breaker rating. I also use primarily induction motors.

Other than that, haze is right. You typically won't see many large (>50 HP) motors on your average MCC. These are negligible when it comes to short circuit calculations. Just check and see what size breakers you have, and that should give you an idea.