How do I check if a single on/off checkbox is checked?
There's a mix of correct and incorrect details here, so I will try to sort them out. This is for Rules on Drupal 7, and applies to working with a Boolean field.

To make a condition that checks a field's Boolean state (true/false, yes/no, checked/unchecked), first make sure that you can access the field in the data selector. As Bojan mentioned above, and as it mentioned on the admin page itself, 


  To make entity fields appear in the data selector, you may have to use
  the condition 'entity has field' (or 'content is of type').


The operator you want to choose is "equals".

For the "Data Value" field, you can input values in two modes, make sure you are using "direct input mode". You should then be able to check the checkbox that represents TRUE for that field.

Translated, what you are saying is, "Is this field equal to TRUE?" You can also negate it by leaving it unchecked or using the "negate" option below.