1. What happens when nobody answers?
The single most revealing question. If an unanswered request eventually opens your apps, the app is designed around not annoying you, and it will let you out on exactly the evening you should have stayed shut.
The answer you want is that silence keeps you blocked. It sounds harsh until you notice it is the only answer that makes the request mean anything.
2. Is there an emergency way out?
There should be, it should work instantly, it should not need anybody's permission, and it should work with no signal.
An app without one is a safety problem the first time you need a phone in an emergency. An app with one that is buried, delayed or rationed is the same problem with better marketing. The right design is an override that always works and always leaves a trace.
3. What can the other person see?
Some apps show a partner a full activity feed: which apps, what times, how long. Decide whether you want that, because it is difficult to walk back once somebody has seen it.
The alternative is that they see only their own part: that you asked, and what they answered, and how long you have kept it up. Warden takes that position, with no app names and no times of day, and it is not configurable upward by the Keyholder.
4. Does the second person have to pay?
You are asking somebody for a favour. If the app then asks them for money, you have handed a friend a bill, and they will decline.
Check that the other side is free permanently, not free for a trial.
5. What happens if you delete it?
Most apps have no idea, and iOS provides no uninstall callback, so no app can know for certain.
What a considered one does is notice the silence. If the phone stops checking in, the other person is told that it went quiet. This is the difference between an arrangement and a decoration.
6. Does it claim things it cannot know?
If the marketing quotes a figure for how much of your day it recovered, the app is presenting an estimate as a measurement, because iOS does not hand third party apps your usage.
This is a good proxy for everything else. An app willing to invent that number is willing to be relaxed about the rest of what it tells you.
Common questions
Are paid blockers better than free ones?
Not automatically. What paying should buy you is that the company does not need your data and does not need to keep you in the app. Check what the free tier gives up before assuming the paid one is better.
Should both people use the same app?
Only if you both want to be blocked. If one of you has the problem and the other is helping, an app built for mutual locking will fit you badly.