If you do not know where to look for the definitions you can use texdef. To use it with LaTeX definitions you have to either call it with the option -t latex or use the corresponding alias latexdef (if the alias is defined on your system).
To check the definition for \labelitemi you can issue texdef -t latex \labelitemi. To also check where the definition is to be found add the option -f. So, to check the definitions for \labelitemi, \labelitemii, \labelitemiii and \labelitemiv you can issue
texdef -t latex -f \labelitemi \labelitemii \labelitemiii \labelitemiv
and on my system it returns:
\labelitemi first defined in "article.cls".
\labelitemi:
\long macro:->\textbullet
\labelitemii first defined in "article.cls".
\labelitemii:
\long macro:->\normalfont \bfseries \textendash
\labelitemiii first defined in "article.cls".
\labelitemiii:
\long macro:->\textasteriskcentered
\labelitemiv first defined in "article.cls".
\labelitemiv:
\long macro:->\textperiodcentered
If you want to see the available options for texdef or learn more about it you can access its documentation with texdoc texdef.