Best way to generate white noise, DC to 1MHz
Noise diodes with guaranteed specifications are available from various suppliers:

http://www.micronetics.com/RF_Components/Noise_Source_Products.html

http://noisecom.com/products/components/nc100-200-300-400-series-chips-and-diodes

A simple way of testing the output would be to feed it into a suitable radio receiver, and tune it across the frequency range. That's basically how a spectrum analyser works.

In many cases, such as adjusting a matching network, a calibrated noise source isn't necessary as relative measurements are sufficient.

Here is a simple noise source of mine, with a broad-band amplifier.