You'll have to find each element within the form by name and check whether it's a checkbox.
But note that it's perfectly valid to have multiple text boxes, passwords, file uploads, etc. as well. Nothing defines a form input as being "multiple" in any way except the presence of two successful controls with the same name when you happen to look at the form.
It might be easier to just deal with this on the backend when you know you need an array. Or submit the form with plain old form encoding rather than JSON, and let your backend's HTTP library deal with it. (Most of them let you do something like request.GET.getall('mycheckbox')
to always get a list back.)