everyone that says thanks is running a script?

No

How can we tell the difference?

A part of the thanks is normal written and different, a part is only copy & paste which is, mostly, shown because the user enter lesser GAs as the autojoiners, write in lesser GAs, enter the same type and same games of GAs (as example RPGs or Injustice 2), the thanks messages aren't sended (always) in the same second.
If you use ESGST you have timestamps with the exact second of entries and comments and when check with it different "thanks spammers", that you see in "each GA", then you see very clear which ones could be copy and paste and which accounts post and enter in lightspeed that isn't possible for humans.

Besides that have the thanks script sometimes a error and post the message 2x.

Such users, mostly, don't react when someone write them something as answer on the message.

Their steamprofiles show, often, comments about "please activate your sg win (i wait since days that you do it...)".

It give more then enough possibilities to see the difference.