You know if you lock him, note the alt, and then he jams.
A jamming target does not necessarily know you have launched at it, otherwise HOJ would be pretty useless. If you launch and crank immediately, however, to a human opponent it is fairly obvious you have HOJ'ed, but sometimes they think you are going defensive on someone elses shot and as you put it, 'fly right into it'. I only use it at over 7000km, when there are two/three lights left on berozya from the enemy radar, just before burnthru.