I've never heard of Skiff. Beyond studying the protocols and system design, here's a couple of things off the top of my head to help:
- Follow the money. Are they charging enough to not be tempted to sell data about their users?
- Who is in charge? Have leadership demonstrated respect towards their user's privacy in the past? See their About Us page
- Read their privacy policy
- Keep up-to-date. Lots of services start out with good intentions, but over time they get acquired, acqui-hired, big investments.. and policies change.