I think without requirements testing cannot be done.

The testing done in this case by doing Exloratory testing 
approach

Exploratory testing is a method of manual testing that is
concisely described as simultaneous learning, test design
and test execution. Cem Kaner, who coined the term in
1983[1], now defines exploratory testing as "a style of
software testing that emphasizes the personal freedom and
responsibility of the individual tester to continually
optimize the quality of his/her work by treating
test-related learning, test design, test execution, and test
result interpretation as mutually supportive activities that
run in parallel throughout the project.

courtesty: http://en.wikipedia.org/wiki/Exploratory_testing