Luckily, being proud of my newly CAA-given capabilities at the time, I added the IFR hours in the remarks column from the outset (I just added another column on the far right).
As far as evidence is concerned, it is your signature in your logbook.
The last three times (IR, CPL and FI ratings) I have given the CAA a printout of my electronic logbook (Excel) which I keep since (a) I am rubbish at adding hours and (b) I also needed to add my hours according to FAA rules, which are completely different. The CAA wanted it signed as correct on each printout page, but accepted it. I always offered them my handwritten paper logbooks as well, they didn't want them.
I have no idea if the CAA checks anything for truthfulness. They could check a lot, though. In my case, I could point to the capabilities of the aircaft flown (all my IFR flights after the rating, other than revalidations, were in turbocharged aircraft, and 60-70% in de-iced tourers), the routes (mostly international, which is much easier to fly IFR than VFR), if they want they can try to dig out the Eurocontrol flight plans for most of them, and even weather reports for IMC hours... but I think that significant "Parker hours" are more likely to be discovered in an interview, or by the FIC instructor giving you the IRI training.