How can I test a circuit breaker tripping current.

Many details missing from that question. Try again with specific information and specific questions about the issue... such as... type of circuit, amperage, what's on the circuit... you get the idea.

Use a Primary Injection Test Set to test the actual amp trip with actual amperage flowing through the circuit breaker.

GE Energy - Primary Injection Testing

Secondary testing will only test the operation of trip units, and does not inject current through a breaker.