Posted: 06/03/2016 at 16:13
Right at the beginning seedlings have two little seed leaves - only when the next leaves appear (proper leaf shape according to the plant type) does it indicate that the roots are growing.
If you're very careful you can prick out at this stage, but it's usually better to leave them until they've got a few more (4 or 6) leaves. For this reason you should sow your seeds really thinly so that they have plenty of room to grow and don't look like cress.
When you prick them out don't ever handle them by the stems - just ease them out of the soil with a plant label, holding them gently by a leaf.
Demonstration here http://www.gardenersworld.com/how-to/projects/video-projects/how-to-prick-out-seedlings/298.html