In Vancouver, for instance, it's very clear. ~280,000 housing units, of which ~25,000 were recently found to be owned-but-unoccupied. Which doesn't mean international owners, but does mean housing is being used not to live in, or even to rent, but as a pure investment - with obvious consequences for the people actually living there.

A very quick look doesn't turn up this number for San Jose, but it's the most obvious way to distinguish price hikes driven by capped housing and rising salaries from those driven by outside investment.

Usually you will find the data you might want is "not collected".

It is not in anyone's interest to figure out how much bubbles are propped up by foreign money.