Evalute that checkbox is checked or not

I have a repeater and in this repeater there is checkbox and dropdownlist.
I am facing problem in making validation on submit.
The validation is (If checkbox is checked than dropdown is also select)
I have applied each loop from javascript than check for each checkbox, if checked than check for drop down. But not able to identify that checkbox is checked or not.. and same for dropdown list(check if chekbox is checked) 
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