如何检查我的Web应用程序的用户是否尝试使用不同的凭据登录？

I'm trying from 3 days to make or find good way to detect if there is a user trying to login my web application with many login credentials or making flood attack, so I can show captcha as example in this cause in my login page, but with no luck.I'm trying from 3 days to make or find good way