There's a Pharacopoeial method, but HPLC(-MS) will probably be more accurate...

http://www.pharmacopeia.cn/v29240/usp29nf24s0_m8755.html

http://www.pharmacopeia.cn/v29240/usp29nf24s0_m8755.html

I used HPLC-ESI/MS in positive mode with an IonTrap-sytem and works well. The quantification of Betaine is also very easy by NMR but there you need more than 100ng/ml.

I think you can also use simple (HP)TLC methods for its determination. There are 14 publications that were listed by CBS CAMAG data base.

http://www.camag.com/en/tlc_hptlc/customer_magazine_cbs/ccbs_database.cfm?method=query