Good question and the short answer is that in most cases you can't.Some older boilers,Gloworms/Mysons used to have a year stamp but now only the manufacturers will be able to tell you unless you know their codes.
If you look at parts of the plastic do you see clock type symbols? This is a mark of when the plastic was made and it might give you a rough idea of when the boiler was produced.
HTH.
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