What are some ways for a preschool teachr to evaluate their children's after a lesson?

Do you mean in comprehension or how/what?

What are some ways for a preschool teacher to evaluate their children's after a lesson

Okay go through the lesson and ask them questions like if it is a story they read... what were the main characters, what roles did they play... good guy, bad guy, rescuer,. what was the main plot of the story,

I guess that is what you are asking.

In addition to what NOhelp4u said about comprehension...

Are they able to demonstrate an understanding of the concept or lesson you were trying to get across to them? For example, can they show you what shapes go together, how animals can be classified, what they would have if they had 3 books and a friend gave them 3 more, etc..
If you are going over the alphabet, can they recite them, can they pick them out of a group by name, can they match the sound with the letter, etc..
Observations and conversations will be your best judge as to whether a concept is understood.