Thanks to ifood.tv for sharing tips to identify rotten mango:
Bad mangos have spots or lesions. You will find that a rotting mango shows brownish-black or black spots.
Check the flesh. If you find it to be translucent or dark colored, the fruit is rotten.
Check the smell. A ripe and fresh mango has a noticeably good smell. If a mango is overripe, it will give out a funny smell, like that of plastic or cleaning fluid.
Moreover, mangos contain a very high sugar content, as already mentioned. Therefore, when it starts to rot, it tends to ferment and, therefore, gives out a scent like that of alcohol, another sure shot sign to ascertain whether fruit has rotten or not.
Check the non-stem part. Mangoes generally start rotting from the non-stem portion. Therefore, start checking the non-stem area of the fruit for any signs of rot. Check the seed or pit properly for any type of spots, which are a good sign of rot.
Check the structure. Hold the mango in your hand and check whether it is solid or really hard. In case the mango is really hard or extremely soft, it is rotten and, thus, should be discarded.