Just check if $_POST contains key with id of correstponding term.
if (isset($_POST['recvid'][$term->term_id])) {
However, I would not loop through all the terms. get_terms
function has corresponding option. So, to receive all posted ids, you would do this (also note, that first parameter may be a string, no need for an array):
$brands = get_terms('recipebrands', [
'include' => array_filter(array_keys($_POST['recvid']), function($value) { return is_numeric($value); }),
]);
What above code does?
array_keys
returns $_POST array keys obviously.array_filter
filters out only numeric keys (see anonymous function).include
option ofget_brands()
allows you to specify exact ids of terms you require.