But why is it so hard to ask? If you're not sure
And there's the rub. What if you're not sure? And no, it's not hard to ask. If you are saying men should always ask, I wouldn't necessarily disagree with it. But don't talk about how hard something is to say when there's no consensus around whether it should be said at all in the first place.
The rest of the sentence: because your partner is a bit quiet/ still
That's very subjective and will be influenced by the man's cultural background and previous experience with other partners as to what he views as normal.
Outright resistance or a complete lack of participation may be easy enough to spot but half-heartedness might not be interpreted in the same way by everyone. I think we need research into that.
It also assumes that women express what they are feeling. We know from previous threads (if nothing else) that some women do not act in accordance with how they feel but sometimes give the impression they're enjoying something when they're not. Unfortunately, pretending to enjoy something in the here and now doesn't mean you want to have sex next (although it would be more likely to mean this if you were being genuine). There is room for a clash of expectations (that might not be discovered until after penetration) if one party assumes that enthusiastic sexual contact leads naturally to penetrative sex while the other is under the impression that no man would do that without asking first. I think as women we ought to sort out what we do expect in that respect, and then publicise it.
Deliberately misleading signals are going to have a significant effect whether the man is 'in doubt' about what's expected. I'm not for one moment blaming women for pretending - it's sad that they should ever be in that position. But obviously, if the appropriateness of seeking verbal consent rests on the clause 'if you're in doubt' (as implied in the sentence quoted), then someone pretending to be enthusiastic when they are not is going to make a huge difference to whether there is reasonable cause for doubt.
This is not as impossible a situation as you're making out.