Welcome in the forum.
Do a UART_ReadRxStatus() API and check for UART_RX_STS_MRKSPC bit containing 0 or 1. See datasheet for the function.
Let's do it the other way: Can you please post your complete project so that we all can have a look at all of your settings. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
will not work as expected. You are testing a constant, but you should test the status bits as
if(RS422_ReadRxStatus() & RS422_in_RX_STS_MRKSPC)
Additionally: Your components are a bit old. Update components to latest (Project -> Update Components). Are you using Creator 4.0 SP1? When not, update Creator first.