One way to check for this condition, is to display the Resource or Resource Assignments data column. With the 'Level Resources' option unchecked (per Options, Calculations), a '^' (caret) will be displayed next to each resource name that is overbooked. With the Level Resources option checked, a '~' (tilde) should be displayed to indicate that the task has been delayed (successfully leveled) to resolve the resource's overbooking.
If a resource is booked for more than it's Max Available on two or more tasks running concurrently, one or more tasks should be pushed out in time (rescheduled) to resolve the conflict and avoid overbooking the resource. (The above assumes the 'Max Available' option in the Resource Spreadsheet is set to 100% for that resource.)
If you do not like where Time Line has scheduled a particular task, you can manually resolve the resource conflict. For example, assign the resource at a lower Number Assigned or Effort the Assignments Info Box. In other words, assign the resource at .5 to two tasks, .33 to three tasks, or .25 to four tasks, and so forth. Or raise the Maximum Available in the Resource Spreadsheet, to allow the resource to be scheduled for more work. Note that a started task (Percent Complete > 0) or done task (Percent Complete = 100) will not be delayed by resource leveling. Nor will resources that are of type 'Unleveled' (per the Resource Spreadsheet) cause resource delay.