Here's a few ideas:
Sow in a band or row. All of your plants of one variety will come up at about the same time. Weeds will stand out in a neat, orderly row of plants.

Google for images of the seedlings you are growing.

Usually seed packets tell you the estimated time for germination. Anything that comes up way before the estimated germ time or way after is probably a weed.

Examine your plants carefully each day and next year you'll recognize them.

Jon

You already recieved good advice. When I was young I lived in an area where carrots were grown in 50 acre plots. The weeds always came up long before the carrots. The solution was when the weeds came up they hired a crop duster to spray a herbicide from the air. A very calm morning was needed to prevent lawsuits from over spray from the neighbors. This method, though effective, was discontinued many years ago. Al

are we talking veggie seeds or plant/flower seeds?

here are a couple ideas:

don't pull anything until it gets up to 2-3" tall, it should have enough leaves by that time to ID what it is. this is generally safe; there are some weeds that propogate via roots/tubers, but generally, nothing is going to seed till it gets much taller

wood skewers ... i use them for veggies, i place one at either end of my row and run a string between if required. if anything is more than 1" out of true with that line, it gets pulled, period. you could do the same thing when seeding flowers and other plants - put skewers everywhere you intend to seed, and plant your seeds less than one inch in front of it. anything to the left, right or behind the skewer should be safe to pull, since you didn't seed there.

find weed pages, like THIS ONE, so you're able to identify the weeds that do come up. Walter Reeves has some great resources at his site for here in GA. your local extension office may have literature to help you with this, call them and inquire.

I have planted seed like sunflowers, sage, basil, and misc. leftover seeds in various beds in my yard. I often forget I've planted things so I started putting in little plastic colored toothpicks to remind me where to water. I also put markers by volunteers such as like some tiny bronze fennel seedlings to remind me not to accidentally pull them when I'm weeding.

The most reliable way to tell weeds from plants is to cut every thing down, the stuff that grow back are your weeds. lol

kandm: "cut every thing down" lol! Exactly what I felt like doing this morning while weeding my carrot bed. So many weeds and such tiny carrot seedlings. I've got to invest in a stirrup hoe.

Jon

You'll get used to what your local weed seedlings look like over time.

Plant a few seeds in a small pot using sterile seed starting mix. Anything that grows there should be from the seeds you planted. Take the pot out to the garden plot when you weed and compare your seedlings to those in the pot. This is especially helpful if you are direct sowing a seed mixture or are broadcast seeding instead of planting in rows.

I think naturegirl_2007 is right on. It shouldn't take too long. After the first true lives are a few days old the pattern of many are already obvious. Checking the color of the stems against the plants in the small pots may help.

Also when you get to pulling note the root characteristics. Some weeds may have a different root and will pull up differently from a desired plant that looks similar.

Some of us had the benefit of grandmothers who knew all about such things. It is becoming increasingly difficult to get such help because our society is so mobile.