I think the question has two answers:
Are they locked from the outside? And are the locked from the inside?
My understanding is that they are actually locked. Here are two links with some information.
First, there’s an interesting bit of lore about the doors on the space shuttle that might shed some insight:
What happens when an astronaut in orbit says he’s not coming back? [Ars Technica]
Apparently the Space Shuttle originally had a handle for opening the door that was found after the shuttle entered use to have a bad habit of instilling a bit of “call-of-the-void”. They eventually added a padlock. Also, it should be noted that these doors are not Star Trek-like sliding doors with a bunch of electronics. They’re much more like submarine bulkheads with big-ass mechanics, as I understand it. This was on the shuttle, but I think the design logic of the ISS was inherited from the space shuttle.
Second is this post on Stack Exchange:
User TidalWave explains how hatches in general on the ISS are not accessible from the outside. They’re opened from the inside. I would assume that some exceptions probably exist for edge cases. They must have had a way to get in the first time, for instance. But by and large, it appears that the ISS is not accessible from the outside.
HAHAHAH