There are few free sites which you can use in mods like IP Tracker these days, geobytes being one of them - it is usually fairly accurate.
Ultimately I use dnsstuff.com to verify checks, and even that has become significantly limited compared to what they used to offer for free - you now have to pay for the extensive service they used to offer, but the Reverse DNS check is still available. There are others available but they are becoming increasingly restricted, largely as a result of abuse.
Server specs (Server OS / Apache / MySQL / PHP / DragonflyCMS):
linux / apache 2.2.22 / mysqli 5.6.34 / 7.1.22 / 10.0.48.9418+