RTC 4.0.1 Traditional Project Plan - how do I easly detect when a user has been scheduled for multiple tasks at the same time overallocating the user?

I have created a Traditional Project plan (iteration phase plan) and based on resource leveling the start and end dates for tasks for this user are correct. I then added a predecessor task to one of the user's tasks and the start date was correctly adjusted to begin after the predecessor task's completion date. However  adjusting the start date caused the task to be scheduled "overlapping" an existing task so that the user ended being over allocated by 40 hours for that week.

1. Shouldn't the scheduler automatically adjust based on the new constrain and levelling?

2. If number one is not true - how do I easily see that over allocation ?

3. If number 1 is not true what is the best way to handle the situation?