When you login to
www.icloud.com on a computer, you'll find various "apps" there, like Contacts, Calendar, Mail (your iCloud account only), iCloud Drive, Pages, Numbers, Keynotes (if you use them in connection with iCloud), Find my iPhone, and Settings.
In Find my iPhone, you can see all your Apple devices and their current location, if they're turned on and Find my ... is enabled. You can play sound, enable Lost Mode and erase the devices from there.
In Settings, you'll see a list of your devices, among other things, such as iCloud storage. When you select one of your devices, you'll find it's serial number, and IMEI (if possible).
Hope that helps.