Before purchasing a domain, I make sure I perform due diligence first. This might help you before purchasing one:
1. Check whois first to make sure if it has been previously registered.
2. Check in Google search, simply key in info:domain. If the result in Google does not match with the description of the domain that you're planning to buy, then it's fake PR.
3. You can also check the backlinks of the domain using Majestic SEO.
4. Check the backlinks of the domain. You should target at getting at least 2 or more backlinks with PR that is equal or higher than the domain you want to buy. This gives you guarantee that the domain will maintain its PR in case one of the links was removed as the other link would be sufficient.
5. Check if it has inner pages with page rank. You can use SEOQuake or Google search (type in site: domain name).
6. Check if it has drops. If it has drops, then it's not a good buy.