I use the same method as Patrick.
One year I had literally HUNDREDS of tomato plants popping up in my garden. I pulled the great majority but the ones that were left turned out to be the fabulous, HUGE yellow tomatoes that were very low acid. They must have ridden in on some
compost material donations from my neighbors, because I had never planted that type of tomato here - large toms don't do so well in the desert. Man were they good! Wish I knew what they were. I got one more year of good plants from some seeds but then they disappeared as quickly as they appeared so the mystery remains...