Finding the pinout is not that hard.
You only need the datasheets of the 16C452 and the MC145404.
https://cdn.datasheetspdf.com/pdf-down/X/R/-/XR16-C452.pdf
https://pdf1.alldatasheet.com/datasheet-pdf/v … +/datasheet.pdf
The 16C452 provides TTL-level RS232 and thus needs the 145404 to translate this to the real rs232 norm (mostly around +-12V)
So you have to see which driver/receiver is connected to which port on the slot connector and then trace the driver/receiver to the uart.
ISA-Bus <-> Glue Logic <-> 16C452 <-> MC145404 <-> external connector
A simply multimeter in diode or continuity test is sufficient.
Just print out the pinouts, maybe mark the ins and outs in a different color and just measure your way trough ...
The harder part is getting to know the IO-ports ans IRQs needed to drive this card.
But there are ways to detect.