These days, most funds are public offer. That means anyone that residing in Australia can join them. Some funds such as Government and certain Industry funds are only accessible to employers of the Government or people in that industry.

The best way to tell is by looking at the fund’s status which is usually listed at beginning of the fund’s Product Disclosure Statement (PDS), which can be found online. Otherwise, email the fund directly and ask them.
SuperSavvy's website hosts a great deal of information on how funds measure up – how their performance, fees and insurance rate, how they compare to similar funds, what are their strengths – and their weaknesses.

There is a lot of information available on super funds. Use it! Just make sure that you are getting that information from a reliable, unbiased source. Such as Savvy.