The number cast or printed on the part you took off is the only thing in this process that cannot be wrong. Everything else, including what the previous owner told you the machine was, can be.
So the counter asks for it first. Photograph the part with the number showing, send it on WhatsApp, and in most cases you will get an answer with a price before you have finished parking.
Where there is no number, or the number has worn away, we work backwards: the machine, the year, the engine size, and then what the part has to sit between. That takes longer and it is still better than ordering by the picture on a website.
There is no charge for any of this, including on the days when the answer is that we do not have it and you should try the shop two doors down.