How do I test if my domain name is now on Cloudflare?

We will provision CNAMEs when you enable CDN through hosting control panel, so you would want to do a ping or traceroute to the CNAME record you activated(generally the www record, such as www.yourdomain.com).  CloudFlare's IPs will show when you do the lookup.

e.g.

C:\Users\Administrator>ping www.yourdomain.com

Pinging www.yourdomain.com.cdn.cloudflare.net [104.28.3.93] with 32 bytes of data:
Reply from 104.28.3.93: bytes=32 time<1ms TTL=59
Reply from 104.28.3.93: bytes=32 time<1ms TTL=59
Reply from 104.28.3.93: bytes=32 time=1ms TTL=59
Reply from 104.28.3.93: bytes=32 time<1ms TTL=59

Ping statistics for 104.28.3.93:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
    Minimum = 0ms, Maximum = 1ms, Average = 0ms