1. You can't do it starting with nothing.
2. Before you do it you have to at least know the following 2 things:

• the rating
• the first letter of there user name

yes, knowing there rating is sometimes enough(you can find the most suspicious one and think it's him) but not always works that way. How to find all the user names with x rating: you can google(users with "x" rating on codeforces) where x is the rating of there real account. What will list: users witch have or had rating of x. Remember: google helps in about 90% times nowadays(got stronger by the years).