I picked some wild blackberries, mashed them up with sugar and water, plus yeast and two campden tablets, and put these into a bucket and covered (loosely, with a t-shirt - that was probably my mistake), as per the recipe. Of course, this kind of thing is very attractive to flies. And when I uncovered the wine, to strain it, I saw maybe a half-dozen flies hovering around. I do not know what kind of flies they were. But how can I make sure that this batch has not been spoilt by this?
IMHO, the only real way to test a fermented product is by taste and smell. If the brew tastes and smells OK then it is fit to drink. If the brew tastes too acidic or sour then it is somewhat "spoilt". How sour that can be is a matter of personal taste. If the brew tastes very slightly sour then it is sometimes possible to recover it by adding some calcium carbonate which can neutralise any acid present. If the brew tastes "strange" or musty then that too can be an off flavour caused by infection. Similarly with smell - very bad aromas indicate infection. However all tastes and smells are a matter of personal preference. I have still to find a fermented product that is actually poisonous.
It is also worth remembering that a young wine can be "less than optimal" in flavour and aroma. Quite often just bottling the wine and leaving it to mature for a year will cure a multitude of problems and result in a wine that is quite palatable.
As a general method fruit wines can be made by pasteurising the fruit in very hot water - or even by a very quick rolling boil of the mashed fruit and sugar. This will effectively sterilise the ingredients. Cool to room temperature before adding yeast. There is no need to add campden tablets using this method and that would be better for the initial yeast growth. The fruit wine should begin fermenting within a day and the ferment should be actively producing bubbles on the surface of the brew. If you intend to make much wine it would be better to get a brew bucket with a lid and an airlock. Of course one can always use a glass/plastic 5L demijohn or other suitable container with lid if one brews batches too small to fill a larger brew bin.