I don't think there's any way of really telling just how long a tablet has been used for. It's not like a car, there's no odometer.
The condition might be an indicator, if it's all dinged-up and bashed, then it's probably had a very hard life. Expected battery life might be a clue as well. If it's not lasting as long as it should, probably means the battery might be almost finished.
You may just have to go on what the seller tells you, whether you trust or believe him/her.