\int_new:N \l__my_tmpa_int
\cs_new:Npn \l__my_ifnum_aux #1 \q_stop
{
\if:w X\tl_to_str:n{#1}X
\else: \int_zero:N \l__my_tmpa_int
\fi:
}
\prg_new_conditional:Npnn \my_is_digit_test:n #1 {p,T,F,TF}
{
\tex_afterassignment:D \l__my_ifnum_aux
\l__mytmpa_int=0#1 ~ \q_stop
\int_compare:nNnTF { \l__my_tmpa_int } > { 0 }
{
\prg_return_true:
}
{
\prg_return_false:
}
}