I'm posting some pictures of thirsty lithops on demand :)
I have just taken them so these are examples of thirsty lithops that do NOT need to be watered. It's November and this is how they should look like this time of the year. No need to feel sorry for them.
However, if you see them like this in the time period from April to September (October, if still sunny), and the surface is soft to the touch, it's a clear sign that the plant needs and should get some water. There is an exception when it's very very hot in Summer and the plants get wrinkly but the surface stays hard as stone - this means they went into a heat induced sleep and should not be bothered. They can be watered when in doubt but they most likely will not react until the heatwave is over so they better be left alone.
Here are the typical signs of a thirsty plant:
1. The sides get wrinkly. Easy to see on those that grow in a longer shape naturally (olivacea or dorotheae or julii)
2. The plant tries to bury itself. This you can see on short-bodies plants (lesliei or bromfieldii). They look like pies with a crust from above. You'll see the wrinkles on the sides if you dig them up a bit.
Of course in Summer you don't need to wait for this level of dehydration and can water much sooner if it's sunny. However, a grown up plant can take it and you can go on a 3 week vacation no problem. Once watered they will return to their normal shape in a day or two.