Essentially you don't, it's confidential information. Most blended families don't adopt one another's children, so if this is in relation to your question about inheritance and intestacy, it's likely unless you KNOW that the older children you discussed were adopted that they in fact were not. You're only real option is to ask them. If they decline to answer you can only make your suspicions privy to whoever is administrating the estate.
I think you ought to take some advice from a specialist. If they haven't been adopted they can't prove a negative. However if you are alleging they have been you must prove.
Your two are the minors. Consequently they may have a claim to a greater share if the estate under the Inheritance (provision for family and dependents) Act 1975.