It will also depend on whether it's a specialist (BMW or ///M) or just any old garage.
History from somewhere like Simpsons, Darren Wood, Darren Farrell (Darjac), Phil Crouch (CPC Engineering), Noel Butler (NCB Autokraft), etc. should be recognisable to anyone from the ///M world - or a quick internet search would let them know that these are well-rated specialists.
History from AA Autocentre, Kwik-Fit, Bill's MOT Centre, etc. would worry me slightly.
My Z4MC is coming up to 80k miles, and it's about time I stopped taking it to dealers where I worry every time whether they've done a proper job or not (and why I UV mark parts to make sure they've been replaced/removed as per the service schedule). Half the time I'll take it to my indy/specialist/mate when I'm next in his area and get him to do a quick once over too.
Be careful of indy's who seem TOO CHEAP, as even those have got to make a profit. So anyone charging less than about £35/hour and working from premises (rather than working on their drive) cannot be doing a proper job. So an oil service with £65 of oil, £10 filter and £35 of labour would be £110 MINIMUM. If they're doing it cheaper than that, or you're supplying your own oil/parts (where they'll also make a little profit), then don't expect a proper job - unless they're one of the places that will find lots of other bits need doing and are only using the cheap servicing as a 'marketing' tool.