You can use the Protparam tool for calculating the isoelectic point of your peptide: http://web.expasy.org/protparam/
When you use a pH lower than the pI then your peptide is most likely positively charged. Higher pH than the pI means negative net charge. Hope I could help with that.
If you just want to know the net charge of a polypeptide, add the number of +ve charged amino acids (arginine and lysine) and the number of -ve charged ones (aspartic acid and glutamic acid). Whichever is the greater number twlls you the net charge. Just ignore the charge on the amino and carboxy terminal ends of the peptide as they cancer each other out.