You don't know that a group is free of spies, but a group is less likely to have a spy than the current setup whereby pervasive, omnipresent surveillance is facilitated by WeChat, leading to people needlessly imprisoned and tortured for the thoughtcrime of others.
WeChat groups are designed to be private (as opposed to being public, not necessarily to protect privacy). To join one you normally have to know either the admin or a member of the group. A QR code for public access is available but it's forced to expire in seven days. So for a regular sized group of a few dozen people, it's relatively easy to spot a stranger.
How do you know you can trust your friends?
In that case you need to pay those spies, while at this moment you just run a database query.
You don’t need to pay neighbors to report one another. They’ll turn each other in over petty grievances, or the expectation of favored treatment by authorities. China’s social credit points seem like a potentially powerful incentive.