Did you know TypeScript's type system is Turing complete?

transform-class.ts

// Return a type with only the methods of T that return R.
type MethodsWithReturnType<T, R> = {
    [P in keyof T]: T[P] extends () => infer RR ? (RR extends R ? P : never) : never
}[keyof T]

// Filter away all keys except methods that return void and
// return a new type with those methods returning booleans.
type VoidMethodsToBoolMethods<T> = { [P in MethodsWithReturnType<T, void>]: () => boolean }

// Our test input class:
class MyTest {
    x = 123

    one() {}

    two() {}

    three() {
        return 123
    }
}

// Extract void methods from MyTest and make them boolean methods.
type MyTestBools = VoidMethodsToBoolMethods<MyTest>

// Proof:
const x: MyTestBools = {
    one: () => true,
    two: () => false,
}