How do jagex know ur real life trading?

Suddenly somebody gives you a santa for nothing?
Isn't that suspicous?

Ya, they can be quite certain. Just by lots of things that they can't necesarily prove.

There are probably many factors that they use. I would say the biggest one would be the odds that one low level guy is just going to hand out millions of gp (or even have millions of gp ) to multiple people for nothing in return... not likely. And they give it to specific people, not just like quitting runescape and handing out to random people right around them. They switch worlds and head to specific people to give them the money.

I know there could be a few circumstances where this would be legit and no rules are broken, but the odds of that are slim to none. And Jagex might even be able to use other things to determine if the person isn't just giving a few mil to each of his good friends.

Maybe they use cookies I'll have to check

It would be obvious if this is true or not because they will probably be low combat lvl (if they are farmers) or high combat lvl (someone quitting). Farmers will not waste their time lvling up their combat when they know they will probably get banned real fast.
Also, there would need to be a history of suspicious trades of this nature. A single occurence won't get someone banned.
Maybe Jagex also keeps track of friendslists. If someone just added someone and suddenly one gives the other cash for nothing would be different than if they have been on each other's friendlists for quite awhile.

I dont think they can , they just say that to scare people into not doing it.

Impossible, when registering or even visiting these kind of sites first thing that pops out is a License Agreement that tells that if you aren't in any way in connected with Jagex you can enter the site if you are and you enter the site you can easely get sued.