easy to check....

run around for a while runnign a tank fairly well down (say to what you estimate is a couple of gallons left...)

now note the distance to empty and average MPG figures...fro mthose you can work out what the computer 'thinks' you have in the tank...

now fill the tank to the top...(note the pumped figure)..now get the capacity for your tank from the owners manual and subtract the pumped figure...and your left with what should already have been in the tank...compair to the earlier calc from your computer...

I sugegst you do it a few times at different gas stations to average out pump errors and any pump that may shut off 'early'...

I did it for the fun of it..and it seems pretty accurate on mine..but i figure on a possible error of a gallon in the calcs (pump innacuracy etc) so given I average about 30mpg I always refil at about 60+ left as a minimum..thus giving myself a little 'in hand' in case i cant find a petrol station etc...

By the way the stuff about crap in the tank etc...very valid points, but if you do a lot of driving where your really pushing fuel through the car i wouldnt worry about it over much...I often go through more than a tank a day and drop fuel system cleaners through the car fairly regularly...so i dont worry to much, but on an old car of my wifes where a tank pin holed in the bottom I must have poured over a gallon of water out of the bottom of the tank...