one way which is kinda slow but effective is using string and tape... say you do a rough drawing whatever size paper... next use the average of your angles to find your vanishing point which lie off the page. its tricky, works better on a large table or a wall if its extreme. I used this method to correct perspectives i may have missed. as long as you keep your points located with tape and where you page sits.. you can come back to it later and work it some more.
hope this helps
draw, draw, erase, repeat