Give yourself and your child identical sets of objects and stand a book between you so neither can see the other's side.
Your child gives instructions and you follow them exactly — 'put the cup on the book' — building your arrangement from their words alone.
Remove the screen and compare. Where the arrangements differ, that is information about which words were missing, not a failure.
Being understood without gestures forces precise, complete language: position words, order words, and the awareness that the listener does not already know what you mean. That last piece is perspective-taking, and it is genuinely hard at this age.