You can find the pinouts for ASUS COM & LPT headers here:E11133_MB_Pin_Definition_v2_web_only.pdf
That URL suggests it's for the Z170-K board, but it's linked from the PRIME B250M-A Manual
page. I'd be surprised if ASUS have multiple different pinout for those headers, so it's likely standard across all their boards that have them.
From a quick search and cross-reference, it looks like the ASUS COM port is AT/Everex pinout. Serial ports should be robust as far as using the incorrect pins, as long as what they are connected to is another serial port, so it's fairly safe.
I'd guess the LPT header is probably standard. The DB-25 parallel port on the back of PCs certainly was very standard, the IBM/Centronics/IEEE1284 pinout, but I'm not certain about the PCB headers.