Is there any way to use tools such as Fiddler to easily figure out if any software from my system sends data to the internet behind the scene?
I have downloaded some open source free software's to achieve some tasks. But worried if they scan my PC and send data to cloud behind the scene!
If the software is indeed open source,your best bet is to download the source code,audit it and compile it yourself.
Otherwise, there are forensic technics that can assess what the software is doing when running but they are more complex to employ correctly.
In both case,you will need (much) more expertise than can be easily written down in a simple post to reach you goal so if you do not have that expertise, you should consult sooth someone who has.
That said, OSS is usually less prone to that kind of problem since it's easier to expose it by auditing the source code. One way to improve your trust in the software is to download the source code at one moment,wait a couple of month to see if any malicious or problematic section is reported on the projects web page or issue tracker and, if none appears, compile that code and use it.
You can use a packet/socket sniffer, like Wireshark. You can analyse internet traffic and find the ports used by the application.
Without much knowledge of packet capture software like wireshark. It maybe better to look into something simpler such as glasswire to see if it even makes an outgoing connection, if not then you have nothing to worry about, else you can look into deeper analysis
Fiddler isn't a good tool for that purpose.
Instead, I would use wireshark (Open Source network capture tool) which will report you every network packets that pass through your machine (including others application traffic, Windows Update traffic, etc...)
Here is the official documentation for starting capture.
