Since you use tags that start with web, I assume you are building a system that requires HTML to be downloaded in order for a paid impression to work.
What you need to do is learn robot behaviour by looking at the server access log files. On a server with apache installed, the file is typically named access_log. If your server is used frequently, you will see hundreds, if not thousands of lines in it. Each line contains the IP address of the remote device that's connected to your server as well as the resource it requested along with the date and time.
Generally, there's at least one second between the time a person goes from one page to the next on a website unless of course the website is a poorly designed high-speed guessing game with no instructions, and even then, the speed might never be faster than one second due to initial connection latency.
If you see the same IP address listed along with the exact same date and time for the next 20+ lines straight, then its clearly a robot trying to attack your system (maybe trying to produce a denial of service attack).
Another way to check if its a robot is to look for the files being requested, especially in the error logs. If you constantly see a similar pattern of files over and over again, especially if none of them exist, then it means one or more systems may be trying to break into your server by assuming you have content management systems installed such as Wordpress.
Also, some robots may be requesting files with screwed up names, or may be falsely identifying themselves. Any line containing source code is likely a line coming from a robot. For example, code containing this:
Because I'm unsure the name of the server (apache? nginx?) is serving the content on your system, I'm unable to tell you which column in the log files belong to the IP address or which belong to the file being requested etc., but when you look at them, especially after you make a request to the server, the patterns may become easier to discover.