I am a beginner fingerstyle guitarist. I watch songs on youtube and try to play them. Sometimes the player has played the song in a different tuning than standard(sometimes even one string is tuned to some other tuning like high E string to D#). How do i find the tuning of string in the song played? Is there an online tool which can help? If not , how can i do by ear? For example.for this fingerstyle arrangement. Can anyone please help how can i do that??
I don't think any kind of software would be able to help. When you can see the performer the job is easier; then it becomes a matter of identifying the pitch you hear and comparing it to the technique you see. For instance, at 0:11 in the posted video, he plays his lowest open string and gets (a pitch near) E flat.* You could similarly look for moments in which open strings are played, or you could look at fretted notes, count the frets, and deduce the open string. You can also use a bit of common sense and trial-and-error to see where a certain tuning puts your fingers; often tunings are chosen because they make technique easier.
The tricky part in this process might be identifying the pitch you hear. The easiest way might be to play a short bit and use an in-tune piano or other keyboard to find the note that sounds the same.
* Note that, in this video, whatever tuning he's picked, the instrument isn't very in tune with itself. I'm also not fluent enough in guitar fingering to tell, but it's possible it's just standard guitar tuning but roughly a half step low.