Using fiddler I can intercept an ajax post request and with request composer I can resend the same request, causing the server to respond it normally. It doesnt matter if protocol is http or https (fiddler deciphers HTTPS traffic), with tools like Fiddler it is just possible.
On web applications side is it possible to understand or prevent such requests? How?
No. There's no way to prevent this. URLs are meant to be accessed. If it shouldn't be accessed, don't put it online or require some method of authentication. Some clients may pass an identifiable User-Agent header that can be restricted, and Fiddler probably does as well. However, the whole point of tools like Fiddler is to be able to make any type of custom request, which includes this User-Agent string. So, even if you block the default User-Agent, there's nothing stopping the user of Fiddler from changing the User-Agent to something that won't be blocked.