You could attempt contacting steam and ask if they know how many attempts have been made from different IP addresses in total for login to your account. I feel like that's really the only way to verify what you're proposing. Steam likely has logs of all the IP addresses that attempt login to whatever account.

I'm skeptical of what you're proposing because it's not hard to design a system that freezes mass random IP login attempts to an account after 'x' low number of random attempts and then only allow the past successful IP addresses to continue with a successful login. As well, as do an email verification if the password is successful but being used from a new IP address.