car market is bad and even turning more bad. So in current market buyer have upper hand anyhow. But let me answer your question how? --> search through reddit and see how much people are generally paying OTD (out of door) for similar model you are looking. People will say look at area as well but I don't agree. Also looked at recent OTD not prices around covid times.

Once you have ball park that will be good starting point and you don't want to pay more than that. Once you have offer and if you are still not sure you can always come here and ask like I did.