The easiest approach to define fair trade is probably to describe unfair trade: child-labor, working more than nine/ten hours each day, working six/seven days a week, no health insurance, no holidays, no sick-leave, no maternity leave, getting paid less than the minimum wage, being exposed to health hazards at your workplace, unpaid overtime and so on. These kind of working conditions are rampant in sweatshops around the globe.
Unfortunately most high street garments which are available today are made in sweatshops. Why is this possible: economic globalization. "The popularity of the "free" market [...] hastened the globalization process. Large corporations are now free to seek out low-wage havens: impoverished countries where corporations benefit from oppressive dictatorial regimes that actively suppress workers' freedoms of speech and association."
So how can one avoid buying products made in a sweatshop? It definitely isn't easy especially when it comes to fashion since the availability of fair trade fashion is still limited compared to the choice offered by conventionally produced clothes. However I want to show that there are beautiful alternatives. I would say that the safest way to buy sweatshop-free products is to buy from local producers hence my "Made in Berlin" posts. Then there are some credible labels like "People Tree" and "Misericordia" (and this list will grow much longer as soon as I have time for more research).
If you want to find out more about working conditions in the garment industry I recommend the clean clothes campaign, Kampagne für Saubere Kleidung, war on want - lets clean up fashion and frequently asked questions about sweatshops.