you can check a websites worth on some sites, however this is not an indication at all of how much the owner is currently making from it.

anyway just google something like "website worth calculator"

Yeah it's difficult to tell without actually asking for proof. Traffic is an indicator (assuming they have ads or affiliate links on their site or have a good product) but even that could just be a lot of non-targeted traffic.

Proof is key. If you're considering buying a site, the vendor should have clear proof of earnings and a breakdown of the income sources.