The seeds you plant will be in rows, the weeds may be in the rows as well but they will also be everywhere else. You can start learning what weed seedlings look like by doing some hand weeding in the areas where you are positive you did not plant lettuce seed. you can also wait about a week after the lettuce has sprouted and than it is pretty easy to tell what's a crop and what's not

boulderbelt, I did think of that and that's probaly what I'll have to do. I was just going to scatter them thickly and cut when small. But it beats spending any more money on seeds.

I learned long ago that you really need to plant this stuff in rows about 2" wide and than have 6" to 8" between the rows so you can get 3 to 4 rows in a 4' wide bed. The rows need to be far enough apart to easily get a hoe in there so that weeding does not consume hours of your day trying to keep the salad bed clean (and they do need to be clean, especially if selling to chefs).

Plus I have found that most of the greens do better with some space on either side. When I have done dense planting I always seem to have portions damp off, leaving holes in the bed and a place for weeds to take hold.