I am a FA for NWA. NWA have two types of 757's.
The 756 (with overwing exits) DON'T have tray tables in the arm rests and the armersts do go up.
The 755 (with four doors on each side) DO have trays in the armrest and they can't go up.
A easier way to tell is if the exit seats are row thirty something its the 755 which have trays in the arm rests. Hope this helps.
By the way this is my first post