If it was painted before the 1970s and if its white, then people often will just proceed assuming that it does have it, since lead was so commonly used as the white pigment in white paint. In that case, you could just proceed with stripping - the goal with either that or sanding is to get a nice, flat, smooth surface that the paint will adhere to . You don't want to see the ridges of where the paint is showing through your final coat.... but you don't need to remove every last bit of paint since you are painting over. Otherwise you can get test kits from hardware store for not a lot of $.

I have a mailbox cover I bought from a vendor on Etsy. After several years, the front could use some paint but not that bad. Anyhow, I emailed the vendor to ask what type of paint to use to keep the present finish. Here is his process:

" they are painted black then white then sanded then dipped in a brown wash."

It pretty much looks like your doors do now! ha But you could go easy on the sanding. Also a bit of work.

ETA: forgot you wanted a black finish, but perhaps you can play around with color and the same technique to get the finish you'd like.

just paint it with black paint. It will still look old. I painted my old fir doors, and they still look old.

I wonder if the OP is after a distressed finish?