Hi!
Our place is circa 1896, we have the same issues, plus our apt is the "plumbing hub"~ so when things happen in other units, it impacts us...
We approach this dilemma in one of two ways~
We do remind guests, with our morning check in message, to take extra care...
based upon the guest..we are either extremely direct or quirky.
You may want to consider using the "biodegradable enzyme/ eating stuff once a month" ...sorry, forgot what it is called... as we were considering doing this, once we check with our plumber..
We try not to worry about it, but are prepared if it doesn't...We have a snake/plunger on hand, as well as emergency plumbing number, and relationship with hotel that can take emergency guests.
On a quick note, no real way to screen guests, after 9 years, we just trust our gut.