Use a volt meter to check each cell as it charges, each cell should never go above 4.2 volts. Much above that is when lipos get exciting, I think 4.25 is the absolute limit, some say 4.22 I think. If they are all the same voltage or at least within a couple hundreths you are good to go. If not, either a bad pack or you need a balancer.

You can try charging 1 cell at a time to manually balance, assuming you charger does 1 cell. You are using a lipo charger right?