I'm a little confused by your question. It is titled "how to check bus current in PSSE." However, the concept of current is poorly defined for a bus as, per Kirchhoff's laws, the sum of the current at a bus is zero.
So I'm interpreting your question to mean "how do I report the current of all lines connected to a bus." First I would define a subsystem around the bus using the bsys() command:
psspy.bsys(1,0,[.12, 999], 0, [], 1, [busNum, ], 0, [], 0, [])
You can then use the bus subsystem API to get the amps of all tie-lines.
ierr, iarray = psspy.aflowreal(1,1,2,1,'AMPS')
If you are also interested in the other devices connected to a bus, there is a similar set of functions for the other circuit elements (amachreal, afxshuntreal, etc..) See Chapter 8 of the API documentation for more info.