I saw marks on my friend’s arm and I’m not sure what to think

I went to school today and noticed my friend had a few straight red lines on her arm - like scratches or something, I’m not really sure how to describe it.

I just want to know how to tell if that could be sh or something else.

I’m not trying to assume anything, I just wanna understand how people usually tell if it’s SH or not. How can I tell the difference?