Well there is no real quick way to find complete list of sub domains for a given website as far as what I know, but with the help of single Google query we could find out how many sub domains a website has and what the names of the sub domain sites are.
Steps to find website sub domain list
1. Access Google Search
2. Enter query like *.Google.com if you want to find sub domain list for domain (website) Google.com
3. Doing this will open up list of subdomain Google.com has along with various other result which may not be relevant for this query, now keep noting down the sub-domain names and keep moving to next search result page until you find couple of sub domain names.
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