Do you have original structural drawings? How much do you know about the construction quality assurance and drawings control? It really depends on the trust you have in the drawings and their implementation. If they're reasonably accurate it may just be a question of spot checking major and/or critical dimensions, poking around and making sure nobody's gone and added significant loads or cut away at anything and making sure there isn't obvious corrosion or other things of that sort.
If you don't have drawings, have reason not to trust them, or find obvious things that don't match you might end up having to go in and measure every property that could reasonably affect your work.
If there are halfway reasonable drawings I'd generally prefer to have a walkaround at the beginning of the project to ensure that the drawings are right in concept, then do a verification of the information used for my design just prior to or during the IFR stage. That way you aren't trying to as-built absolutely every part of a complicated structure but you've done your due diligence at the beginning so you aren't completely blindsided and you can actually fully investigate the important things later once you have a full plan and know what matters.