I'm considering buying used slick tyres. When buying normal street tyres I'd normally look at the depth of the tread to know how many kilometers I can still do on them. However tread is non existent on slicks. How can I know how much rubber is left on used slicks?
"tread is non existent on slicks."
If the tires you are getting have non existent tread then they are already worn.
Tread is the rubber that is added on the carcass. The depth of the grooves will indicate the tread height.
Tread is not groove. Don't confuse the two.
A lack of tread grooves or tread patterns does NOT mean that there is no tread.
A serviceable slick tire for example is a tire with tread but no grooves or pattern.
On a typical tire with grooves, you can measure the tread or check the wear marker bar.
On slicks, where there are no grooves, you can't measure the tread, so other means are provided, such as the "drill hole" looking wear indicators.