This is the list of cars - https://www.game-automobilista2.com/vehicles From what I can see there is no way to tell which are DLC
DLC cars: Formula USA Gen1 featuring Reynard 95i & Lola T95 Formula USA Gen2 featuring Reynard 98i, Swift 009c & Lola T98 Formula USA Gen3 featuring Reynard 2Ki & Lola B2K/00 DPi Class (featuring Cadillac DPi-V.R) GTE Class (featuring BMW M8, Corvette C8.R and Porsche 991 RSR)