Polarising filter is a must, to either reduce or enhance the reflections you get.
If you are at street level and aiming up, you are mainly going to get clouds and tops of other buildings as reflections. If that doesn't work, you might try to gain access to nearby buildings, or take telephoto shots from nearby hills for instance.
If you do want strong reflections, the best time to do this is often when the building is in shadow, but the reflected objects are in full sun. This way the glass is dark and the the light reflected buildings/clouds have a lot of contrast and drama.
Pink/red skies around sunrise and sunset can be spectacular reflected in glass buildings.
You may find there are few good angles for photos with good reflections. Once you find these, you may find it's the wrong time of day. Suggest you scout out possible angles and note when the light may be good for those angles. You may have to revisit a few times to get all the best shots. I've taken a lot of shots of glass buildings, a few were easy, most took a lot of legwork to get a decent shot.
If you don't have a great wide shot, try taking some detail shots of just one or two panes/segments of the glass which do have something interesting reflected in them.