I just receive my jeans and they do not appear to be authentic lucky brand,how can tell for sure the jeans i received are not knock-off?

181's have been discontinued on Lucky Brand's website for months now, so if you're getting 181's in 2020 or after they are likely either the last remaining stock or simply not genuine.

are these 100% cotton as in no elastic/ stretch ? i have been looking in stores and everybody is only carrying jeans with stretch

No. They are stretch. I tried buying them and then had to return them. I then went to a Lucky Store and the manager told me Lucky does not make 100% cotton anymore. Only the part stretchy kind. Hence why they are now going out of business. My guess. Hahaha. I tried ordering 2 different styles off Amazon that said 100% cotton and returned both due to the stretch factor. Total bummer.

does this have two belt loops in the back?

There is one in the center. There is one on the back left side and then you have the Lucky brand logo that you can slide the belt through on the back right side. Hope that helps! These are extremely comfortable. I wear a belt also and don't have issues with the belt going over the top of the jeans in the back.