Best way depends on the town/city/environment/country - and the size of restaurants/franchise/etc.
Here are a few ways.
- Association & townhall meetings - [You get a few at the same time]
- Having a meal there on a weekday
- Getting to know their main suppliers (Fresh food, wine resellers, et al)
- Through a good real estate agents who cover the areas for high traffic business locations
- In some places, there are placement agencies for waiters/waitresses/kitchen helpers - these placement agencies will usually deal directly with the owners/decision makers on staffing.
- IT companies that sell point-of-sales systems to restaurants