If you want to test your cell line to prove the cell line is really HepG2, you should do STR (short tandem repeat) analysis. Every cell line has a spesific STR Profile and you can check it with PCR, if you have the primers. I share a link of ATCC for HepG2 cell line below. You can find STR Profile of HepG2 in the specification section.
I totally agree with Cigdem Ozen. you also can stain different markers of HepG2 cell but you need additional experiment like specific STR profiling by PCR.