documentclass{minimal}
usepackage{hyperref}

begin{document}
begin{Form}

CheckBox[name=ch1]{Yes}

end{Form}
end{document}

documentclass{minimal}
usepackage{hyperref}

begin{document}
begin{Form}
CheckBox[checked,name=ch1]{Yes}

end{Form}
end{document}

makeatletter
Fld@checkedtrue
makeatother

documentclass{minimal}
usepackage{hyperref}

begin{document}

begin{Form}
makeatletterFld@checkedtruemakeatother
CheckBox[name=ch1]{Yes}
CheckBox[name=ch1]{No}
end{Form}


begin{Form}
makeatletterFld@checkedfalsemakeatother
CheckBox[name=ch1]{Yes}
CheckBox[name=ch1]{No}
end{Form}

end{document}