http://gerla.net/gallery/v/misc_2/tomatoes/

http://onfinite.com/libraries/841442/617.jpg

http://i5.photobucket.com/albums/y168/gorgar1/DSCN0764.jpg

There is no good way to know that they are getting burned while it is happening, other than if you are watching very closely you may see the leaves start to look a little grayish and paler than they were when you first set them out; move them to shade immediately if you see that. The full extent of the damage doesn't really show up until a day or two after exposure.

I agree, take action immediately. I grow my seedling indoors in south facing sliders where they get alot of sun but even only a few hours of morning sun outside can burn them. This I have learned from experience. Bringing inside grown plants outdoors can be a shock to them for other reasons as well. I now try to gently introduce them to the great outdoors by placing them in an area that gets just an hour or so of early morning sun. I honestly thing aquainting them with a bit of a chill and some wind is what is important and what they respond to when they become stronger. I love seeing them toughen up.

I just put some tomato and pepper plants outside today to try and harden them off. I won't be able to check on them until I get home from work. They're under an eave of the house, but they will be getting afternoon sun. I did give them lots of water this morning. The temperature is only suppose to be around 70 degrees today. I hope they don't get burned.

Has nothing to do with water.
And afternoon sun is more brutal than morning sun.

Transplants that come from indoors should start with 1 hour of morning sun on the first day. Then 2 hours. Then 3 hours. You can try putting the plants somewhere that they'll only get morning sun and you can put shade cloth over them to reduce the % of sun that gets through.