The front one is the most common one to go, and easy to swap.
Best way to check is leave the handbrake on, put it in gear and slowly raise the clutch whilst a mate stands and looks over the engine. When the clutch bites and the engine looks like it's about to leap out the bonnet then the mount will be shagged.
Drving along you will proberbly get some vibration when you pull away if there really knackered. Otherwise, you'll be able to feel the engine moving around when you pull away and also if you drive slowly but in a high gear.