Usually with a cracked manifold if standing outside of the car (or when driving it), when the engine is accelerated, you will hear a distinct exhaust leak. Depending on how big the crack is, the exhaust leak can be loud. It may also sometimes sound like a "raspy ticking" noise - but again, it's dependent upon how big or small the crack is on the manifold. The exhaust leak noise coming from a cracked manifold will also increase w/ engine RPM's while driving.
It's definitely a distinct and audible (annoying) noise. Pop your hood and with the engine running, grab the throttle, increase the engine RPM's and try and pinpoint where the noise may be coming from within the engine bay.