[underlipton]

How do you find out about the audition?

Also (though perhaps closer to a privilege issue than a nepotism issue):

How did you find the teacher who helped you get ready for the audition?

How did you "pay" for the practice time to prepare for the audition?

[patmorgan23]

Many orchestras will hold open auditions. They'd post them on their website and probably a few regional music target outlets/websites.

Also networking, if you do gigs in the same region the orchestra is located you're bound to bump into a few of the members.

You can also just email the orchestra and ask when they hold auditions.