I've known two ways. Both involve clicking on the "see details" on the stay in question, usually day-of or day before.

1) Sometimes I'll get an error saying something like "cannot display your reservation". That means that the hotel has modified it, and the website can't access it. The new spg.com may handle this more gracefully, as I don't remember seeing it for a while.

2) I see that the room type has changed. I typically book a standard king, and at my current property (westin copley in Boston) I usually see it change to jr suite before I arrive.

As always, it can change back to your original room type of the hotel fills up and needs the room they've upgraded you to for a revenue guest.