I usually judge of the quality of service recieved. Unlike many place except in Doha people don't tip as often as they should.

Now probably back home when you got out and do the shopping someone bags your shopping with a smile (and not a grumble) walks you to your car with a smile (and not a grumble)then you tip them well. Personally... if they just to do it for the sake of getting a tip off an expat and you hand them over 2 riyals you'll still see tham standing there waiting for more?!