Tell diff between html/non-html email owners?

Hello,

After setting up a php-email script I realized that non-html email owners where receiving a bunch of garble html coding as well in their emails…

Is there a way to tell if their email is html capable or not… I am using

$headers = "MIME-Version: 1.0
";
$headers .= "Content-type: text/html; charset=iso-8859-1
";

but that doesnt help with non-html emailers.

Thanks

~ Lacuna