I've to write a Simple validation in my Custom VF/Apex code where a formula field (Grand Total which depends on 3 other fields) should not exceed a set number. If it does I would write a condition in my Apex code & add a error message to page. 

My Issue is that I think formula fields are only evaluated once the record is saved & when the page loads again & that's the reason I can't check for the modified Grand Total field in my apex code when User saves a record. 

Any workaround this , can this be achieved through After Trigger ?