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So I'm creating an app for a specific website, which has comments on certain articles/entities or whatever. I already know how to fetch those comments with RSS, but I'm also curious if I can implement a reply functionality from the app itself. The site doesn't have an API for that, but I know that when you write a comment, it sends a POST request with the contents of the form.
Is there a way to find out what that POST request looks like, so I can send a similar one from my app? The end goal is to be able to reply/post new comments from my app to that website.
Any thoughts? Just in theory.

Is there a way to find out what that POST request looks like, so I can
send a similar one from my app?
Use some debuger, like FireBug for FF or DragonFly for Opera.
so I can send a similar one from my app?
It would be not secure if anyone could see the nature of requests and make such requests in their apps. Apps may be bad and send spam.
There should be some Captcha or CSRF protection.
If there is - you plan will not work.
If there isn't - it is security hole.
My advice.
Better create some API an call it from your APP.
Simple HTTP, XML-RPC, or SOAP.
This will be more secure and nice usefull thing to know.

Look at the page which sends the post request. If the HTML code or javascript is simple you can understand which is the POST data sent to the server. Otherwise maybe you have access to the server and can look at the code there?

Point your browser with Chrome or Firefox (with the Firebug plugin) and open the inspector (Control-Alt-I / Command-Alt-I), then the "Network" tab. There you'll see real-time requests and responses to the server.
If you need to implement authentication from your app, google "cookie jar" and your language.