Gents, having caused my fair share of cock-ups and having recently moved to the LHS I can definately testify to the difficulty of a) speaking up when things are wrong and b) hearing a statement that things are wrong when it is 'incorrectly' couched. My advice is 2 fold. Firstly CRM has got absolutely nothing to do with being nice to each other, nor has it anything to do with being warm and fuzzy. Secondly facts is facts and words are just words.
So what am I saying? basically always make your statements as facts and never as 'veiled suggestions'
i.e. 'Captain/Bob/Bill we need to de-ice',
not 'do you think we need to de-ice?'.
i.e. 'Bob/Bill/Capt we are not stable'
not 'you '500',
him 'stable',
you 'erm....?'.
Present your fears and concerns as statements of fact, Confirmational Bias will nearly always lead an individual to confirm his previous assessment. Any question that is couched in such a way as to allow him/her to do that will cause him/her to do it. So you need to absolutely stop it, by making a simple statement of fact that absolutely denies them the ability to re-evaluate their previous decision using their previous assumptions.
This can be done in a non-hostile, friendly, 'we' way, that still allows the other to make the 'right' decision' without you robbing them of that ability. For example the 'Captain you must go-around' is a frighteningly difficult thing to say to a very senior training captain as a lowly new FO. But 'Capt/Bob/Bill, we are not stable' allows that senior training Captain to still make his command decision to go around without you taking it off him.
Cheers
BJ