Way to know a site's earnings.

Is there a reliable/semi reliable way to know how much money a site generates. Apart from checking it on flippa if its listed.

No.

You can calculate how much that site worth. But you cant know the earnings unless you are the owner.

Unfortunately no since you are not the owner.

well you can get an estimate... look on the site and see their monetization ways.. are they using adsense only, do they sell their own space, are their content ads? if a site is selling actual products then there is no way to know earnings.. but if a site is running off adsense and stuff like that.. then by experience of your site you can get a semi broad number...if you're in a similar niche, then use your rpm and calculate it over the page views they get.. other than that there is no way to know.. if you buying site on flippa, why dont you ask for a up to date earnings sheet..also what is it selling for? depending on the price, you can also get a rough number on what their making..

give a try hiring some one on fiverr. go to their request a gig in my order section.

You can guestimate, but that's about it.