To check if your account is on margin (or borrowing) money, just check your cash balance. You can view your balances by clicking the blue Balances drop drown, located immediately to the right of your account. If your cash balance is negative (in parenthesis), then that means your account is on margin securities (borrowing money). In the example below, this account is margining $16,991.67 in stock. 

Accounts on margin are assessed interest daily (including weekends) and are charged monthly (mid-month). To learn how to calculate a margin debit balance, then please click here