You can tell if it has it by going to the Software tab under the main menu and seeing if it says Full Self Driving is an "included package". However, you can't tell if it's supposed to have it, which is different. Specifically, many cars that pass through Tesla's hands and end up at an auction (i.e. previous owner trades it in to Tesla, Tesla decides not to sell it as used and instead sends it to an auction) have FSD stripped off. However, at least once, the car never received the message from the mothership to turn off FSD. So it appeared to have FSD, and the intermediate dealer sold it to a customer claiming it had FSD, but the terms of their purchase from the auction (and from Tesla) didn't include FSD, so Tesla later removed it after an audit found that the car hadn't been updated as intended.
This is probably rare, and Tesla did make things right for the purchaser (even though it was the intermediate dealer's "fault" in my opinion for not verifying what they were purchasing and re-selling). However, it's worth noting that it's still a risk. Even if the car seems to have FSD, there's always a possibility it will get yanked if it wasn't supposed to have it. I suspect this is overblown, but this is why people tell you that you can't tell and should assume it doesn't have it unless you're certain of its progeny.