How would you say "everything is at her mother's", as in, all the stuff is over there ?
You can't say "у неё матери есть..." because "у неё" means 'she has'. It would be like saying "у меня матери есть...". On the other hand, if её is used to mean 'her x' as it is here, it does not gain the consonant н, which only appears when it functions as 'she' / the genitive form of она
So how con I figure out the difference between "у её матери есть всё" and "у её матери ест всё"?
У её есть means 'she has' but У её ест is meaningless. 'She eats' would be Она ест
I'm guessing that almost all (if not actually all) the times you hear the "У [а] есть [б]" construction, it will be about possessing something, not eating. So there's no worry about confusing the two, since the other one would never be said.
(By the by, what letters do Russians use most often as variables in algebra?)
"у её матери ест всё" = "(some guy) is eating everything at her mother's".
There is actually a distinct difference between ест and есть in terms of how they sound, "т" is softened in "есть" (in colloquial speech с and т both are softened)