You need standards with known molecular mass.
SDS-PAGE separates proteins according to their size (molecular mass) by electrophoresis on a polyacrylamide gel.
One of the lanes must contain a mixture of proteins of known molecular mass to serve as standards.
Your protein should be as pure as possible but, if it is the major component, you can still distinguish it from the impurities.
SDS-PAGE separates proteins based on size.
The concentration of the protein will affect the appearance of the band, but not its position on the electrophoresis plate.
The image above shows the SDS-PAGE result for the purification of glutathione-S-transferase (GST).
Lane M contains the molecular mass markers. Lane 1 is the crude mixture. Lanes 4 and 5 show the purest forms of the protein.