Q: How would I know if a certain transaction about rent would fall under Prepaid rent or rent expense?
A: Look at the period (year) the rent expense is for. Usually in an accounting question they will give you the rent paid for a number of months. Some of those months will fall in the current year 2009 (for example), and some will fall in year 2010 (for example). Rent amount in 2009 is rent expense, rent in 2010 is prepaid rent (asset).