It's not "a scripting project", it's "a script". Scripts don't need to be complicated things. You can start with something like:
on *:TEXT:*:*:echo -a UTF8: $isutf($rawbytes)
If your goal is to get an answer that requires you to do no scripting at all, I've already indicated that mIRC does not support this builtin-- if you truly have something against writing a script, you're using the wrong client. FWIW, I don't know any client that would tell you what encoding a line has without some sort of non-standard customization. The answer is going to be "you have to script it" no matter how many times you ask.
As to why I've never done this before... I've never done such a thing because I don't need it. You do. Therefore, you're the one who should script it. It has nothing to do with easy or hard. Note that for UTF-8 checks, the script is easy. If you want to find out what codepage a line of text might be, the script would be much more complicated. I never said it was going to be easy though. I just said it was possible.