To see what Late Blight looks like on different parts of the plant with a descriptor, click on the "Managing Late Blight in Tomatoes" link on the above page. You will need Adobe Reader or a similar program to see it.
Late Blight used to only be prevalent South of Wisconsin, but it has finally made it up North and seems to be here to stay. If you want to find out if Late Blight is affecting crops in your area, or about other plant diseases, I suggest contacting your local Cooperative Extension Office. You can find yours here: http://www.csrees.usda.gov/Extension/ You'll want to look under "horticulture" when you get to the extension homepage if there is nothing right out in front. Also, local greenhouses and garden stores that sell tomato and/or potato plants, should have information as well.
Once the plant gets infected, it is very difficult to control the spread meaning you have to completely destroy your plants. The anti-fungal products that work against Late Blight do best as preventatives. Their success at control is limited, and they cannot get rid of the Blight all together. If you have Blight infected plants you should pull them completely out of the ground and put them in a plastic bag to go out with the trash. DO NOT COMPOST INFECTED PLANTS!
If your plants are still disease free, then you will want to get a fungicide spray or powder. They have Copper based products (like Bonide Copper Fungicide) that work well and are considered safe for use in organic gardening. There are also other products like Bonide Fung-Onil Multi-purpose Fungicide that will not only prevent Late Blight on tomatoes and potatoes, but will also prevent and/or kill a bunch of other fungus based plant diseases on several different plants. Either way, you'll be good to go!
PLEASE take the necessary steps to prevent the spread of Late Blight. Your tomatoes (and neighbors) will thank you!