How to find BWA version?

Well, from SAP BW side not that easy :-) however everything is possible since there is a class CL_RSDDTREX_SERVICE.

For little bit more comprehensive information about SP of BW as itself you can call method COMP_BI_SP_BIA_REV.

Related posts:

How to check whether your ABAP NetWeaver is HANA DB based?

    Learn your BW in hard way