And either stop to write it down, or do something cunning like take a fly-by photograph. Don't commit it to memory on the assumption that you'll get the pen out at the next convenient stop, because by then your cycling brain will have had time to muddle it and you won't remember if the date on the bridge was 1875 or 1785. DAHIKT.
If you're riding in a group, don't trust the others to have got it, either, unless you're absolutely sure. Otherwise it tends to go the way of informal group navigation.
ETA: Crosspost with marcusjb, who evidently has a better brain than I do.