How does a book determine if you're a sharp?

[Performify]

Cat is incorrect.

Bodog (and most other sites) deal sharp lines by account, regardless of IP. Your behavior there gets flagged as sharp (primarily betting primarily the underdogs which are shaded towards) and that account is flagged as sharp. If you clear your cookies and log out you can get the square lines again, until you log in again.

IP-based tracking is a less than effective way to do differentiation, based on the prevalence of regional ISPs who dynamically assign from a pool of addresses. And the ease of getting a new IP, as you mention

Not to say if you're consistently accessing the sportsbook from two separate accounts from the same IP they won't also notice -- the online security teams are relatively active for the sportsbooks, just less so than the poker side.

-P