If you are not mocking the service itself, you would need to do something like this (be aware of passing the proper parameters when using metaClass:
def "Test"() {
setup:
def calls = 0
service.metaClass.innerMethod = { p1 -> calls++ }
when:
service.doSomething("notexisting@test.com")
then:
calls==1
}
and if you are mocking the service,
def "Test"() {
when:
service.doSomething("notexisting@test.com")
then:
1 * service.innerMethod(_)
}