detecting supported opengl extensions

[almos]

You do:
if (!GL_ARB_texture_float)
{
//float textures not available
}

in GL/glew.h:4423 there is this:
#ifndef GL_ARB_texture_float
#define GL_ARB_texture_float 1

This is decided at compile time.

A runtime check looks like this (copied from the doc you linked, my link was the 2nd hit of google for "opengl detect extension"):
if (glewGetExtension("GL_ARB_fragment_program"))
{
/* Looks like ARB_fragment_program is supported. */
}