You must answer the insurer's question to your best knowledge, but you are not expected to carry-out forensic investigations... if you bought a second hand car and you were unaware of the wheels not being original, then that's fine.
As for mods, simply ask the insurer, to remove doubt. That's what I did when Alfie retrofitted COMAND on my car, and Aviva said no problem - in fact they even said they'll cover it under the existing policy.
There are a few issues with not declaring mods, and they have been covered before, but the result is that the insurer may claim that due to deliberate misinformation on your part, they were unable to correctly asses the risk and calculate your premium - meaning you did not pay the correct premium for your circumstances and your insurance is null. This will also land you in trouble with the police for technically driving without insurance...
Keep in mind that the insurers do not make their risk calculation algorithm public. In the same way that they may have worked out that say teachers have less claims than say builders, they could argue that their statistics show that people who modify their cars are more likely to make claims. If you keep material information from them, they simply can not work out your premium correctly.
Of course this is a worst case scenario, these things will only get looked at if you have a major claim.