You actually can't. If you don't trust the DM, that's it, game over, you are done, this will not be a fun experience, no matter how many manuals and restrictions and rules you throw at them. A DM you can't trust is a DM who you can't trust to obey the rules of any edition no matter how clearly they're spelled out.
Don't mistake the rules for one of the goals of play. The goal isn't to use the rules. The goal is to have a fun time. The rules are only useful in as much as they enable that. There is no neutral arbiter of what is a fun time for everyone. If a DM wants to nerf a rule that's actually fine because they don't think it's fine, that's working as intended, because that DM is going to have more fun that way. And if the players can't trust the DM not to nuke things that are fun for the players, they should have a different DM.
DM freedom is a flaw you say? So people making the game the way they want to play is the wrong way to play the game?
DM freedom is a flaw for me.
It's a feature however, and probably a good one. It's impact on me in practice is fairly negligible, but in principle it's one I oppose.
Truth be told I much prefer playing a game than living out some narrative fantasy. I much much prefer pathfinders approach than 5es - in principal. I hate THoM games, and I hate DM styles who just let players do whatever is creative, regardless of the rules. I love RAW, and I love RAW mastery.
5e unfortunately for me happens to be a lot more fun however than pathfinder, which is old, clunky, and tedious in practice, but I'll always be an advocate for a little DM power as possible, as I view the DM as another player at the table, first amongst equals, but a flawed human still.
I firmly believe that sacrificing some "simulationism" for consistent set of boundaries increases the satisfaction of all players long term, as they're guaranteed a consistent set of boundaries to work with, and cast rest assured with *total* confidence that any achievement (or error) is purely their and theirs alone.
Despite what other posters here think, they're not being consistent across All Things. They're sacrificing consistency for an abstract line in their head that crosses some sort of "realism" line.
This is just an endless debate in play style, which I can accept. What I don't accept however is that there is some RAW justification for certain forcecage rulings here which I am opposed to. There simply isn't. It's play style pure and simple.