I guess the simple answer is they don't. Often though if you are pulled over and they suspect that the trailer is "iffy" in some way, that is all they need to remove it from the road, the onus is very much on YOU to prove that things are in order rather than VOSA/Police to prove that they are not.
I suppose they are instigating these laws now, knowing that to an extent they are un-enforcable to make life easier down the road, there will come a time in ten years maybe or however long when the "I built this shining new trailer before the IVA deadline" simply isn't feasible anymore!

At present I guess they have to rely on honesty to an extent.
On a related note (and not having a pop at anyone on this thread!), I do think that if people spent as much time and effort complying with the applicable laws rather than trying to get round them then things would be better all round. In fairness if a trailer cannot comply with the current IVA test, then it really shouldn't be on the road as the rules are pretty basic and a lot of the technical stuff is available on the Internet already regarding tongue strength and nose weight etc. VOSA are very helpful people too, they are there to help as much as enforce.