Unless a the promoter has specifically mentioned that there will be door sales, there is no guarantee you will be able to purchase a ticket at the event.
It is always best to purchase your tickets ahead of time to avoid queues, avoid an elevated door price and to ensure you don’t miss out on a fun time at the event.
Most event promoters will list a door price is available. If there ins't one, then it's likely door sales won't be available.
For events that are being ticketed by Eventfinda, a "Sold Out" notification will be posted if all the tickets are all gone.