var BS_CHECKBOX = 0x2
var BM_GETCHECK = 0xF0
var res = Interop.Call("user32", "SendMessageW", PlusWnd.GetControlHandle("yourcheckbox"), BM_GETCHECK, BS_CHECKBOX, 0)
if res = 0 .. the checkbox is not checked
if res = 1 .. the checkbox is checked
if res = 2.. the checkbox is "Partial Check"
it works for me