1. Think of a better (realistic not idealistic) alternative.
2. Assume the person you’re about to criticise has already thought of that alternative.
3. Assume that the person has some really good reason (or info that you don’t have) to have chosen against that option.
4. Ask them what that reason(s) is.
Then, please, please DO criticise!!
Most critics don’t even do the first. Some do come up with alternatives, but they assume you couldn’t have thought if it. Very few are gracious enough to ask why you’ve made the decisions you have.
I love those people ;)