how do I know whether it is dead or not.
Nip the very end from a few branches and look for green, living tissue. If the branch is brown and dry, or shriveled in appearance it is likely dead. If many branches are like this try nicking or scratching the trunk itself. If there is green under the bark it may recover.
By your statement about the bubble wrap I assume that you are in an area with colder winters. You probably stressed it by putting it out so late in the season. While Chinese Elms are fairly hardy trees you must realize that dormancy is a process that begins after the summer solstice and continues up until the first frosts when the process is complete. It might be that your tree was not prepared for winter.