This is one of the most common questions we get when we're closed and cannot usually respond before the event.
For events run by the Students' Union (Citrus/Flirt/Freshers/End of Year) there are usually a limited number of tickets on the door. At the worst case these will be sold at midnight once everyone with a ticket has arrived and we know how much space we have to sell. You will need to arrive early to be in with a chance, but we cannot guarantee you will get in.
For events run by societies or external promoters you will need to ask them direct, try the event facebook page if there is one.