Sorry if there are any mistakes here or this has been asked;

I understand that one source of resentment for the Qing was that they were not considered truly Chinese by many, being Manchu. How exactly are Manchurians and Han different? Is there a solid dividing line or is it a rough classification based on language and/or culture moreso than historical distinction?

In Qing dynasty, Chinese people (ren) were divided by Banner (Qi) System: Qiren or non-Qiren. A Qiren was not necessarily to be Manju, but a non-Qiren was definitely not Manju. Qirens didn't need to work to make a living, they could get stipends from the goverment; non-Qirens must work hard or beg to survive. If you lived in such a society, you can easily tell who was Qiren, who was not, especially in the lower class people.