A method I found on http://www.apug.org and am using since then goes like that: take a 5 – 10 cm piece of the film you want to test. It shall be completely exposed to light. Take 50 – 100 ml of the developer you would like to use and pour it in a glass. Put half of the film in the developer and start the clock. First, the part of the film in the developer will become lighter and lighter. As the time goes, the developed part of the film will get darker. At the moment when wet and dry parts get similar at their density, stop the clock. The time, in seconds, multiply by 20. This is you development time for this film and this developer in seconds.