You could check the Toshiba site for a battery check utility. You will always get the best results from a utility created by the manufacturer.
Otherwise fully charge the battery and then remove the charging cable and see how long the laptop will run after a full charge.
Laptop batteries usually degrade if the laptop is continuously charging them. The battery needs to fully charge then completely empty before being charged again. Since that is not always possible you try to follow the guideline as best you can. A laptop battery's lifespan is usually 3-4 years for a decent capacity.