Smell it. If it smells like vinegar, it likely is turning/has turned into vinegar. If it doesn't smell like vinegar, it might just be a lactic bacteria infection. IF it's a lactic infection, you can save it. Does this sound familiar:
"Oiliness or Ropiness: The wine develops an oily look with rope- like treads or strings appearing within it. It pours slowly and thickly with a consistency similar to egg whites, but neither its smell nor taste are effected. The culprit is a lactic acid bacterium and is only fatal to the wine if left untreated. Pour the wine into an open container with greater volume than required. Use an egg whip to beat the wine into a frothiness. Add two crushed Campden tablets per gallon of wine and stir these in with the egg whip. Cover with a sterile cloth and stir the wine every hour or so for about four hours. Return it to a sterile secondary and fit the airlock. After two days, run the wine through a wine filter and return it to another sterile secondary. Again, this problem, like most, can be prevented by pre- treating the must with Campden and sterilizing your equipment scrupulously."
Or does this sound more accurate:
"Acetification: This isn't the worse thing that can happen to wine, but it usually means the end of the wine itself. Acetification is simply the formation of vinegar. If not caught very early on, the process cannot be reversed. But at least you end up with vinegar, which is useful in itself. If ever you detect the slightest smell of vinegar and recognize an acid taste, add one crushed Campden tablet per gallon of wine and stir it in well with a sterilized rod. Wait 24 hours and restart fermentation with a fresh, vigorous yeast. If the smell of vinegar is pronounced, you cannot stop the process.
Acetification is caused by any of several the vinegar bacterium, most commonly Acetobacter aceti, Acetobacter pasteurianus and Acetobacter peroxydans. If you make homemade vinegar, commercial vinegar bacteria can easily contaminate a work area and thereafter contaminate wine if the area is not suitably cleaned after vinegar production. Wild vinegar bacteria are most often introduced on the skins of fruit used to make the wine, during primary fermentation when the must is uncovered, or by using equipment that has not been properly sterilized. It only takes one vinegar bacteria to contaminate an entire batch of wine. When using whole chopped fruit or fresh berries, flowers or leaves for winemaking, always kill off the wild microorganisms by adding one crushed Campden tablet per gallon of must, wait 24 hours, and then add your wine yeast starter. Keep the primary covered at all times except when stirring, checking specific gravity, or adding ingredients. After the initial fermentation in the secondary dies down, always keep the wine level topped up. Vinegar bacterium need air to grow, so the less air in the secondary the better. Finally, always sterilize your secondaries and racking tubes very well. At least two minutes of exposure to sterilizing solution is required to properly sterilize equipment."
Hope this info helps.
Regards, GF.
