Look for promotional booths, and ask for their plan.

Also, ask for 7 day risk free trial. I previously checked and it could apply to Maxis and Celcom, which you can get full refund if not satisfied with their service within 7 days, if subscribed during promo period. Didn't check with any DiGi promo.

Another factor in choosing my telco, is the vicinity of their service center from my home and work. I'm referring to SC's that can do 'big' things like change line, issue new SIM card and stuff. Some branches, to my knowledge, doesn't do all that.

YMMV. Good luck.