On the ACT and SAT, you may be asked where or if a sentence should be added or subtracted from the passage. To determine whether or not a sentence belongs, it is important to read and understand the passage as a whole, not just the part immediately around the sentence. You may also be asked to determine the best placement of a sentence or paragraph in a passage.
Read the whole passage.
The placement of the sentence within the paragraph should make organizational sense.
Is the sentence relevant to the paragraph? Is it relevant to the passage at all?