Steady Altimeter increase. The word "Steady" added to avert any false illusions of climb as opposed to momentary small zoom climb due to over-rotation.
The Original poster alluded to the VSI (pre IVSI) being the most unreliable instrument due to time lag. That's actually a point in favour for the VSI, as, if it is now showing a climb, that climb has been in effect already for several seconds, again, a Sustained climb.
So, Steady Altimeter increase (albeit ever so slow) plus a steady VSI positive indication will jointly give a good assurance of positive climb (IVSI could also be subject to momentary small zoom climb due to over-rotation).
If you have it in your kit, add a sustained positive FPA/FPV indication to the confirmation process.
Regards,
Old Smokey