check if user registered on the required website
<p>i am currently developing a website where user needs to signup on particular websites to get some points.user will register on that website and then enter his user-name and email in our site to tell that he registered on that website with following details and got this user-name. But the problem i am facing is that, as we know we can't directly access another domain's database. So I am currently facing problem to find a method by which i could check if user signed up on that website or not. 
please suggest me something by which i can check the genuineness of user's claim of signing up on that website. (I am developing this website using CodeIgniter 3.x)</p>