You could start by checking the MIME type:

#!/usr/bin/env perl 
use strict;
use warnings;
use LWP::UserAgent;

my $ua = LWP::UserAgent->new;
my @urls = (
    'https://jsonplaceholder.typicode.com/todos/1',
    'https://www.perl.org/'
);

for my $url (@urls) {
    my $res = $ua->get ($url);
    print index($res->header('Content-Type'), 'application/json') > -1
+ ?
    "$url is JSON\n" :
    "$url is not JSON\n";
}
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