A see a site that i want to buy that has 10,000 uniques per month, but how can i check if that is legitimate traffic and not shit traffic?
Alexa doesn't always show accurate traffic details... your best bet is to ask for a screenshot of his stats.
You cant really, if you can you could always whois the domain and find out the owners email adress and ask him if he can provide proof of the stats such as a screenshot.