i have an xt660x 2005, now i wonder if i have a full exhaust system of leovinci x3 or just the slip ons, i see models with some kind of sensor going in the left exhaust pipe, i do not have that.

The exhaust sensor, is it on all year models?
How can i reckon its aftermarket or original?

The sensor came in on 2007 bikes.

It's not used on your bike but handy exhaust to have if you plan on running a power commander auto tune.