Currently the domain check is done via simple DNS lookups. It's not the most reliable way, but it's very fast. Although there is a small probability of checking result errors, for the MVP I decided to stop with this solution.

I plan to apply for TLD Zone Access programs (like http://www.verisigninc.com/en_US/products-and-services/domai...) if this project gets a positive feedback. That would help to keep things both fast and reliable.

Probably using DNS and not whois. DNS is very fast and has no restrictions but can be inaccurate.