There are two types of exposed exterior doors, those that leak and those that are going to leak.
Sure, there are exceptions if your exterior doors are under an overhang or face north, but most doors exposed to weather are unsafe. There are a few things you can look for and adjust to make sure your home is free of leaks.
The Threshold. Doors that have missing thresholds are bound to leak. Thresholds can be designed to be on the outside or inside and are often installed backwards. Outside, look for water collecting on the top of the threshold and causing wood decay in the base of the jamb material - this is common on exposed doors with the metal thresholds and softwood jambs. Is the threshold sloped to drain properly? Is it level? Does it flex when walked on? Does it bow?
The Hinges. Make sure to have exterior rated hinges. These hinges cannot have the pins removed unless the door is halfway open; therefore they are secure. Make sure they are made of stainless steel or brass so they are resistant to rust.
Sill Pan Flashing. These are critical in keeping materials below an exposed door dry. You most likely won't be able to see these since they are installed below the threshold.
How to Test Yourself. Open the door and look at the base of the door and jamb for water stains or wood decay. Is your flooring inside cupped (assuming you have wood floors)? If you have carpet inside look to see if the tack strip or subfloor is stained or damaged.
I hope you find these tips helpful!
Devereaux Van Dyne, Certified Professional Home Inspector