Issue a test.

"I have a proposal for Batman. I don't feel you are an appropriate substitute in this case."
"I can handle anything he can!" She demands, indignantly. "Tell me what it is!"
"It's a test of skill. If he were to pass, I was to offer him unparalleled aid in his crusade."
"I'll take the test in his place!" Batgirl insists.
"You will not like the conditions of the test."
"If it was designed for Batman, I can handle it. Just tell me what it is."