If the dog have underlying issues whether they be healt of behavioral you are not very likely to pick these up when viewing,
For example the dog COULD (not saying it has) have a sensitive stomach,, you could ask about diet and toilet habits ect but you only have the owners account to go on. Also you will more then likely be told the dog neither chews, soils the house nor barks excessily, again this could be something that would only come to light when you have the dog.

Don't please take this the wrong way - the dog could be perefect - try to the the interations between dog and owner,, i.e does it look scared/relaxed/confident/agressive. Also you should ask if the dogs been vaccinated.

will comment more when know the breed

Be sure you find out what the dogs current diet is as any change would probably upset him