Although a home inspector will be able to determine most of the failed seals, there is no guarantee that they can all be identified. Over the years I have done many inspections where the failed seals are readily apparent when I get to the home at 9:00 AM, and then by 11:00AM, the same warmed-up windows show no condensation or fogging. The home inspector/engineer can provide an excellent basis, in terms of the overall scope of glass replacement needed.
The degradation of the insulating qualities of the windows is overstated. The energy savings reaped by fixing the failed seals (usually replacing the glass units), pales in comparison to the cost of replacing the glass. The economic payback is long and energy savings cannot justify the repair. That is why failed seals are basically a cosmetic issue. No one wants to look through a foggy, streaked window. Manufacturer's warranties should be closely consulted (via the current owner), since these can sometimes pay for the glass replacement (at least materials) if the windows are less than 10 years old, and sometimes when they are even older.
If you know the failed seals are a widespread problem in the home, I would recommend contacting a glass repair company that can replace the glass units without replacing the entire window. They will have the best idea regarding which windows have failed seals. They will also be able to give a repair cost estimate. Having the glass company at the home inspection, as a specialist, would be the ideal situation. Otherwise, they can be called in after the inspection, based on the inspector's recommendations, to provide a quotation.