How to find deployment task which are assigned to a bamboo agent

This is indeed not explicated on Dedicating an agent and slightly non intuitive given the page name, though reasonably straight forward as such:

1. Go to the Agents management page - all dedicated agents are marked as such:
   
   
2. Go to the agent in question by clicking its name or View.
   
   
3. Click the Dedicate agent button, which happens to also be the dedication management page:
   
   
   
   
4. Click the Delete icon on the specific dedication you want to remove:
   
   

You can go to Administration > Agents.

Click on View link against Agent name in a list of agents. Then click on Dedicate agent button. There you can see list of jobs and deployment environments assigned to agent