To me, hiking is totally different from the rest of the activities on the list. A hike is just walking. The other possibilities require a lot more technical skill, gear, trust, and experience. In those cases, the most important thing to get a feel for is what the other person's experiences have been. For example, if you're looking for a partner for Ixtaccihuatl, someone who's done Shasta and Kilimanjaro probably has both the necessary snow skills and some relevant high-altitude experience.
In some cases, you might be better off looking for a partner at the location of the activity rather than online. For instance, if you want to do Whitney, you can probably easily turn up prospective partners at the hostel in Lone Pine.
You can try to get a feel for what the person's reputation is like. For example, if you're working through meetup.com, you can tell whether the person has led trips, whether the people who came on those trips left positive comments, and whether the person has been an organizer in the group for a while.
You can try to get some feel from the person's online persona and interactions what the person is like. Do they brag about that time they needed a helicopter rescue? Do they act like they need to prove they're cool? Do they spend a lot of time talking about how much fun it was getting drunk with their buddies around the campfire?