I've never been able to discern the signs (if it ever happens). It's really a disparity of vigour in the engagement; the way to manage it to tell the SH that as there is this disparity, he will be unable to put the ball in until you are satisfied that the scrum is over the mark - which may require some to-ing and fro-ing. Free kick the feeding SH if he ignores this instruction. The onus is now on the more aggressive pack to either ameliorate the power of their engagement, or else having "got a nudge on", to de-nudge before the scrummage starts.
Question - doesn't this unwarrantedly depower the aggressive scrum? No - it is a legal requirement.
[LAWS] 20.1(a) Where the scrum takes place. The place for a scrum is where the infringement or stoppage happened, or as near to it as is practicable in the field of play, unless otherwise stated in Law.[/LAWS]
[LAWS]20.1(j) Stationary and parallel. Until the ball leaves the scrum half’s hands, the scrum must be stationary and the middle line must be parallel to the goal lines. A team must not shove the scrum away from the mark before the ball is thrown in.
Sanction: Free Kick[/LAWS]
When you hear "They aren't taking the hit, Sir", what you are really hearing is: you've blown the whistle because the scrum has splintered/fallen over/travelled a mile, and we don't want you to ping us for the 20.1(j) offence we just committed.