The best way I've found is using Yandex's Image search. You can do the same with Google and Bing but Yandex seems to be more comprehensive.
First you need to save the image you are curious about. Then go to:
and click the camera icon on the right end of the search bar. On the next screen click Select File. Then select the photo you have downloaded and it will search the internet for it. Lucky for us, scammers often use the same photo on many dating sites. You may see your girlfriend on a lot of other sites, all with different names, often from different locations. My don't these poor girls ever travel a lot!
The main goal of these girls/scammers is to convince you to send them money every month. They can do this with many guys so they're actually pulling in quite a haul, even from western standards. Many are dirt poor with needy children and if they've been scammed by foreign guys before they may be less likely to resist this temptation, especially if they're exceptionally attractive and its really hard to find a decent job where they are.
Rather than send them any money put that money into the bank and save it to buy a plane ticket to see them in person. If you spend a lot of time with them you'll probably know if what they've been saying is genuine.
Another test is it to "meet" them from another account and see how she acts. VPN browser extensions allow you to change your IP address and if you delete your cache and cookies there's no way she'll know its you. Use a different browser and use that to access your email if you communicate that way.