How can I find out which universities allow a sandwich (stapler) PhD thesis?

The department where you are going to submit the thesis probably has a document of PhD/thesis regulations that specify if such a thesis is allowed. For example, my department's regulations have the following statement:

[...] Scientific publications may form a part of a doctoral thesis. If the doctoral thesis consists of several scientific papers, a presentation of the guidelines of the papers submitted has to be added in an appropriate extent. [...]

i.e. a sandwich thesis is possible here. So you should find out if your favored departments have similar regulations and have a look in there.

I know it's a "personal" answer, but I don't think it depends only on the university, but also on the department. For instance, I did my PhD in Computer Science at Paris 6, in France, and the thesis had to be an original document (although it's clearly possible to use content from published papers, but not in a copy/paste way). But I knew someone who did his PhD in Physics at Paris 6, and his PhD was a sandwich/stapler thesis. So there doesn't seem to be a global rule across the University, at least not for Paris 6.

It's essentially impossible to know if you'll be able to do a stapler thesis or not, because the decision also lies with the thesis supervisor, who may not allow you to do that, even if it is allowed by the rules of your university or department. I was co-advised, and one thought a stapler thesis was an inherently logical way to write the thesis, while the other thought it was a really bad way to write a thesis, and didn't allow his students to do it.