You seem to have an UI based approach. You have UI components talking to each other. One UI component asks the other UI component “tell me what you know”.
I like a data based approach, where all the forms deal with the same data set. They show the data and they tell the data about any user interaction, like a click on a checkbox. The UI components show and interact with the data, not with other UI components.
This way not only can you know about the status of your objects more easily, you can also change your UI components, remove or add forms and nothing breaks.
In your case, knowing very little about your implementation, I would have the checkbox events add the checked info to the data. Perhaps your data is a table row and you don’t have that “checked” column in the table, but you can still add it to it from code. Then, any other UI component could check whether “checked” has been added or not and what is its value.
In my more complex apps I create a class that holds the row in a member and adds any other feature, like your “checked” or “selected” properties, that does not exist in the table. In simpler apps I just add it to the row object.
In this post I describe this technique (and more). It is not a post for beginners and I don’t know your Python skill level. You can give it a try and see if it helps. (I know, you asked for a simple “how do I do this little thing” and here I am telling you to redesign your whole app… I just hope it helps, maybe on your next app 
).