How can we check any file is password protect or not in C#

You can't: password protected data is encrypted (or you wouldn't need a password to access it) and without the correct password it just looks like random numbers - which is exactly what a random data file can look like. Yes, you can sometimes say "this is a text file" and maybe from that say "oh, it's CSV data" or "it's JSON" - but even text files can look like random data is they are written in Cyrillic, Korean, Chinese, Arabic, ... the list goes on. And binary data like EXE files, DLLs, images, music, video can look pretty random as well, unless you understand the actual data format or each of them (and there are probably dozens of different types of each). Then you get to proprietary data formats, where the format detail may not even be published so it just looks like random data and may or may not be encrypted.

To make matters worse, Excel and Word not only support password protection, but have modern compressed XML based files (which look pretty random to the casual eye even if unprotected) but also support the older, proprietary binary formats as well!

I'd rethink what you are trying to achieve so you can either find another way to get there, or to limit the scope of what you need to detect - this is probably not going to be a success in it's current form.