Lacto Fermented Vegetables - Off Flavors
You need the bible of lacto-fermentation 'the Art of Fermentation' by Sandor Katz.

That aside, the general rule is: if it smells and tastes appetizing then it's fine. If it has green, blue or black 'mold' on it, chuck it out. White is alright. Botulism is actually very rare with fermented vegetables. Fermenting is very safe compared to other preservation methods.

Sometimes I get a yukky whiff at the moment I open a fermenting crock, but then once I put a forkful on a plate for a minute it is fine and yummy. So don't decide in the first few seconds. 

The best advice I can give on what are the prime factors for lacto going right (tasty healthy) rather than wrong (slimy inedible waste of food) is: use organic veg so you know the right bacteria is present from the start (or add a coupla organic cabbage leaves, they are foolproof). Also get your salt ratio right in the brine, it kills off the baddies. Consider the texture of your veg. If you are cutting/grating it finely, it will likely ferment hard and fast (the bacteria finds lots of surface area). Ferment theses for a shorter time than chunky or fibrous veg to get good complex flavours. Last tip- invest in a good crock with an airlock 'moat' on it and weights in it. they are pricey but really improve your strike rate a lot.

On flavour: veg that has been fermented 'right' has an unmistakable pickles tang at the start, then a lingering complex savoury musty ending. No matter what veg you use.

Oh, and cucumbers (gherkins) are widely held to be tricky to perfect, especially hard to keep crunchy. Beginners usually steer clear. Good first experiments are carrots, cabbages and cauliflower.