Read the rules at Racing-Line and GT3Times before you decry them.
Most of the drivers who have submitted "top" times have been previously vetted - by means of an Xport/Sharkport or by physically viewing the replay. Those that haven't tend to be questioned - usually ending up with them either proving the time or chucking a complete wobbly.
With Flinx, mr.p, bigp/bigpsycho, Max E., Arco/Wiese, jmr, Rod/Rotty, Didi, Honeycar, bobkart, Missing_Link, Arwin, daan, Thrasher et al. you can be as close to 100% sure as you can be that the car is as it is meant to be in the game and that the run involved no smashing off walls or driving over grass. There's always a small doubt margin - where drivers such as those do something unintentionally - like adding the wrong part for an online race, or having a fraction of a second where a third wheel is off-track - but I trust those guys' times without question.
The current series - GTLeague at GTRP - has random driver replay checks every week. Drivers must submit a replay by Xport/Sharkport. These runs are then watched for cleanliness and analysed through mk's for car legitimacy - it is impossible to create an undetectable hybrid. The trust has been built up over time, but even trusted drivers - including many on the list above - are asked for replays each round.