Hi, you can recognize serious, big and reputable agency by many indications. Of course, it will spend more money for advertisment, so look at its logo, registration, participation in Regional, National and International structures, style and language of announcement etc.
I think the opposite to Najnaj. Any clowns throw money at an advert, and they think bigger is better, but it is not. If I see companies wasting several thousands on an advert they have to recoup these costs through the prices of the properties and I don't want to pay for their big adverts. It is the people who don't waste money in adverts and us media relations to be commentators in articles that are the cleverest. Personally I think large adverts looks desperate, not very clever, so paradoxically they are not a good advert for that particular company. Avderts a very blunt instrument to market property.
It is the small classified ads where you find the best deals. I have even found some fantastic off market investments via free trade newspapers/websites.