The “forced wort” test basically tells you whether your wort is stable and free from contamination. You take a sample from the run-in to your fermentor, before you pitch any yeast. You need a properly sterile flask for this – merely sanitised probably won’t do the job (although I haven’t tried) and could well give false positives – and you need to take the sample in as sanitary a fashion as possible. Then you simply put the flask somewhere safe and warm (on a shaker if you’ve got one – I haven’t) and check it every day for haze, bubbles, or off smells or tastes, for the next few days to see if anything grows in it. Ideally, it should still be clear and stable at least 7 days later. Don’t put it on a stir plate, as constant stirring will make the remaining proteins go cloudy (I know, I’ve done it).