No, you can not. It does not matter what she would have done in the past; since she has passed away, she cannot authorize you to write out or cash the check. Furthermore, while you say the account is still active, it should NOT be: accounts are frozen when someone dies, then the money is only distributed to the estate, put into a separate estate account for the payment of bills & expenses and eventual distribution to heirs, when the estate's personal representative brings proof of death and their authority to the bank. No one may legally draw checks on a dead person's account, and the bank should not pay out checks on an account one the account-holder dies.