There are two issues here. First, whether you can act for your mother and stepdad and, second, how do you stop them from making bad decisions. The first is easier than the second. The power of attorney document should specify how to establish your stepdad’s incompetence, but if it doesn't, a letter from his doctor should be sufficient. Of course, there could be a problem in getting that letter if the doctor feels that writing it would violate your stepfather's privacy or if your stepfather hasn't seen his doctor recently.
Once you get the letter from the doctor, you will need to present it to all of your parents' financial institutions. That should give you access to their accounts and some control. However, a durable power of attorney does not take away their right to act for themselves. That can only be done by going to court to get appointed guardian or conservator. Whether that will be necessary depends on the circumstances, but you should probably consult with a local elder law attorney. To find an attorney near you, go here: http://www.elderlawanswers.com/elder-law-attorneys.