Is there a foolproof way of getting the right fit?

Am a little hesitant about a recent purchase. Bought a pair of slim fit low rise trousers from H&M. It fitted very well in the changing room.

That was then.

Tried it on this morning, and it looked a little "off". Maybe I need to test it over the weekend first before wearing it to work...

Which brings me back to the title. Are there ways to improve your chances of getting the right fit when you try it on in a store? I know for shoes, the usual advice is to shop for shoes in the afternoon, after our feet have swelled a little.