Lots of reputable breeders advertise online, you can't blame the internet because some people are disreputable. The only way to know for sure is to phone and chat to them and see how they seem. If they seem genuine go and have a look but be prepared to walk away if they seem dodgy or the dogs living conditions are filthy. Straw bedding in a kennel or an outhouse means nothing but the cleanliness does and so does the smell. Take someone with you and be prepared to say sorry the dog isn't what I'm looking for if necessary. The worrying aspect is if they are giving the dog away for free is that unscrupulous people will be willing to take the dog and either sell it on or use it as a bait dog which really doesn't bear thinking about, but people do give up dogs for free for a variety of reasons, they may just want a good home for it rather than make a profit. Not everyone will be willing to give the dog to the first person that comes along but some will, you'll only know that by talking to them.