1. You can have list of questions and corresponding answers in your database.
2. Ask any of the simple question randomly in the registration page(Ex. Show one string ask length of the string, Show some numericals and ask which is grater etc...)
3. Provide the text box/option box for the user. If the answer is correct you can decide it is not a robot.
4. The other way is to create random number and show it as a image in the client side. And ask the user to enter the number. In the process you can validate that.
Following is the link. You can refer my following article for more information
You can fine the sample code to create captcha
Direction is important than speed
After Registration you will get Site Key and Secret Key, then add code given in below link
hope it helps
Editor, DotNetSpider MVM
Microsoft MVP 2014 [ASP.NET/IIS]