But how can you tell if cables purchased online are "quality thicker cables"?
Buying from a reputable brand would to me be the most obvious. Especially cables that are advertised as braided, reinforced etc.
However, the most important thing you need to know when buying a usb-c cable is the correct amp rating. You need to know the amp draw from the charger you are using, and you need to know that your usb-c cable matches or exceeds it.
For example, the Samsung 25W fast charger is a maximum 3 amp charger. Therefore you need to get a usb-c cable that has at least a 3 amp rating. The 45W charger has a 5 amp rating and needs a usb-c cable of at least 5 amps.
If you are buying a usb-c cable, the amp rating needs to be clearly stated. If it isn't, don't buy it. If you buy a usb-c 3 amp cable and pair it with a 45W charger, you won't get the maximum fast charging.
If you go the other way and buy a 5 amp usb-c cable and pair it with a 25W charger, you'll get the full fast charging, you'll just have paid more than you need.