If the same fixed beacon is used then the checksum doesn't vary.
Used mobile then it will vary as the GPS position changes. The same with messages.
APRS Messenger uses a CRC-16 Modbus checksum and there are many code samples on the internet allowing you to generate a checksum in the language of your choice.
APRS Messenger is written in Visual Basic 6 and I can post the code routine on here later today.