Check the daily times guide each morning, it will give you an idea of who to find where. An example would be "Pooh & Friends from 11:00am to 5:00pm Critter Country", so you would know it would be Pooh, just not sure if it would also be Tigger or Eeyore or piglet.
Ask at City Hall for a specific character and they should be able to tell you a time and place.
Try a character meal. You'll get princesses at Ariels. Minnie and friends at Minne's (we also saw Chip, Dale, Tigger, Capatin Hook, and the Fairy Godmother). Goofy's kitchen will have an assortment as well, including Goofy and Pluto. Not sure about Mickey's. When we went it was Lilo and Stitch's breakfast. Storyteller's will have Chip & Dale (we also saw Koda, Kenai, Terk, & Meeko).
HTH.