Regarding H, C and O. Look at those values for abundance. There is only one dominant isotope, the other isotope is totally insignificant for the mass spectrum, so no need to memorise those ones.
Regarding those other elements, as part of your general knowledge in Chemistry, you should be aware of the natural isotopic mixture present in boron, magnesium, potassium, chlorine, bromine and zinc. Also probably uranium too, because of its technological significance for atomic power.
How do you know which elements are isotopic mixtures?
Go look up the Periodic Table. Any element that has an atomic mass that is significantly different to a whole number is probably an isotopic mixture. For example, the A-number of potassium is 39.10. It is a mixture of isotopes 39 (93.3%) and 41 (6.7%) with a tiny bit of 40 thrown in.