when do you know that your dog is tired of walking?

Once this last summer I walked my son to a play date about 3 blocks from our house. I took all the kids and dogs and we stopped by the park on the way home. At the end of our street there is a low spot where water always gathers and there was a nice sized puddle there where the curb and street meets. Alex decided she had had enough and just plopped down in the puddle and refused to move for at least 5 minutes. It took everything I had to finally convince her to get up and walk the block back to our house.