The reason for a lead test is the off-chance that someone used a piece of CI to melt lead for casting into bullets or sinkers or whatever, and that left a lead residue. Maybe someone else knows better but I have never heard of needing to test for lead in the actual casting / metal composition.
If you need to do the test, that depends on your level of paranoia. The possibility someone used any random piece to melt lead is very unlikely but non-zero. The more you know about a piece's individual history, the more confidently you can assess the risk.
If you do choose to test, the best timing would be after stripping it, but obviously you can't do that at point of purchase. So, at that point, you can either take your chances or you can test and take your chances.
Either way, you can find simple lead test kits in the paint isle at the home improvement stores, aren't too expensive. But not free, either.
EDIT - I will say that I believe the buyer who did the lead test was more likely going off of theory than practice...