reference old or prior field value in apex trigger

To reference the old value of a field in a trigger, use the following syntax: 

Trigger.oldMap.get(account.Id).fieldName 

So to ensure a field was changed during an update trigger you will have: 

if (Trigger.isUpdate) { 
if (account.fieldname != Trigger.oldMap.get(account.Id).fieldName) { 
// Field has been changed! 
} 
}