look for the sucker lips and the whiskers
Ask lots of questions, try to get many sources of pictures, videos etc, if you are concerned, your gut is usually your best judgement
Well! Try asking him/her two things: to get on Skype, and his/her phone number. <br />
If he/she refuses get on video on Skype, means they don't wanna be seen! (that raises a red flag there)<br />
Knowing their phone number, mean knowing their location/what country they are living in. For instance, if the person tells you he/she is in France, his code number should show a French code number!
Skype will sorted the lookers from the catfishes. ;)
They'll avoid skyping or calling you. Even after talking for ages