Being overweight can lead to a lot of problems: diabetes, heart issues, certain types of cancers, fatty liver etc.
For a quick test, use the BMI calculator others have suggested. It's not 100% accurate, but unless you're build like The Mountain or The Rock, it will give you a pretty good idea of whether you're obese or not.
There's also the method where you measure the fat around your belly and the circumference of your waist, some scales will also measure body fat (again they're not 100% accurate, but they'll give you an idea of where you lie). And of course you could visit your doctor or a dietician.