This is what the trees look like:

This is what the seeds look like:

 

The best time to look is in the spring, when the purple/pink flowers cover the tree. In the summer the leaves, heart-shaped, but fatter than long, are the easiest way to recognize them. In the late summer (August) the green flat seed pods appear, and in the fall the seed pods turn brown.

I have left these descriptions vague so you will have the fun of finding the trees for yourself.

There is a very nice redbud (see the above pictures) in the gardens at the Wilcastle Center just north of Wilmington off of Pennsylvania  Avenue. There are several small trees on Lovering Avenue by the Plaza, and a larger one on Delaware Avenue just north of Trolley Square, in front of a famous tavern. There is also a good sized tree  across from Lincoln Camera.

This (2003) was my first time trying to grow these seeds, and it wasn't very successful. I collected them in the fall, after they were brown and dried, but still on the tree. I stratified them over the winter in my unheated space, then planted them without any scarification. After reading a little, I think they need to be kept moist over the winter, and perhaps scarified before storage. 
For the 2004 season I tried scarifying and stratification; read about it here.

 The Plants Database on the web.

An article about redbud in the Southern garden.