Most of the 1.7.2 non-vanilla servers are using the hacked 1.6.4 spigot non-supported update. Keeping this as simple as possible, 1.6.4 unsupported spigot has been 'hacked' to work with 1.7.2 minecraft. But it's not a true 1.7.2 server and the way you can tell is to look for any of the new blocks... you wont find them.
Here is a test you can do within a few minutes of joining a 1.7.2 server with plugins. Just grab a yellow flower, grind it to yellow dye, smelt 8 glass from sand, then try to make the new yellow tinted glass and one of two things will happen: 1) you wont be able to make the yellow glass or 2) you will crash the server. A spigot server works the same way as a bukkit server and supports the same plugins, this is because spigot is basically a fine-tuned version of bukkit. Ref: http://www.spigotmc....i/about-spigot/
If you try the yellow glass test and you actually get yellow glass, then someone has managed to make sense out of the 250,000 relevant lines of code (out of a million lines altered) to do something truly amazing... beat the MCP team. Or, the plugins they're using are not bukkit plugins and are not effected by changes in the newest version of MC.