I'd be interested to know if anyone has a better one.

Huevos, perhaps?

so would I

No rule of thumb. If only learning a language were so easy. Generally SER is the essence of a thing while ESTAR is the state. Locations normally go with ESTAR and events with SER. Passive voice is always SER. If you listen to Spanish enough in the end you just know instinctively what is right.

yes - we know that

and it's what I tell my students

and eventually you do know instinctively

but for beginners................every little helps

and not everyone has the luxury of being able to listen to Spanish people very often - not everyone who studies spanish lives in Spain an dhas the opportunity to hear Spanish outside the hour or so lesson every week

So no rule of thumb and even the "generally" includes a "normally".

Maybe the "rule" posted earlier wasn't such a bad guideline for beginners, after all.