If you signed up directly for Cloudflare:
Cloudflare's IPs and nameservers will appear in lookups. If you do a ping or traceroute to your domain, Cloudflare's IPs will show when you do the lookup. Lookups on something like Whois will also show our nameservers in the lookup.
If you signed up through a Cloudflare hosting partner:
Cloudflare can only provision CNAMEs when you signup through a hosting partner, so you would want to do a ping or traceroute to the CNAME record you activated at one of our hosting partners (generally the www record, such as www.yourdomain.com).