My employer promised to pay me for my trial day at work, but she didn't...?

She said she'd pay me after the trial day, as it was not like my first day at work, it was just to see how it goes. However, she did not pay me, how should i bring this up? Should I wait until my first day this week and see whether she pays me then? I really need money intstantly though.
Update: She pays everyone every two weeks in cash, no cheques.
Update 2: I work in a sandwich bar.
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