How do you find out if someone else put a tracker on you?

From: AirTag - Apple

"AirTag is designed to discourage unwanted tracking. If someone else’s AirTag finds its way into your stuff, your iPhone will notice it’s traveling with you and send you an alert. After a while, if you still haven’t found it, the AirTag will start playing a sound to let you know it’s there."

What to do if you get an alert that an AirTag, Find My network accessory, or set of AirPods is with you - Apple Support

Myname6 wrote:

how do you find out if you have a tracker put on you from someone else?

Short of a careful search of yourself and your accoutrements, you can’t.

Apple AirTags will notify nearby Apple devices, but the wide variety of cellular trackers and such do not have any such notifications.

Various cellular carriers offer cellular-capable tracking tags for sale, as do third-party communications providers, and used-car dealers (among other entities) routinely install trackers in vehicles with loans.

AirTags are probably among the least concerning of this whole vile situation, given the notifications Apple devices receive from nearby AirTags. Other widely-available tracks do not provide these notifications.