Thanks Ralph, one question though

Hi, you look it up in the process configuration of the project.
If you have something like the display name, you would have to use the API to get all available work item types in the API and iterate them to compare the display name with the string you have. 

Thanks Ralph. What I need is to query work items of type "Issue" using the Expressions

It should be easy enough to open the project area, go to the process configuration tab, find the Configuration Data>Work Items section, select Types and Attributes, select the issue work item and look up the type name like in the picture below. This depends on the process template you use.

 Thanks Ralph. I was trying this programatically with the help of https://jazz.net/forum/questions/93841/how-to-get-workitems-list-of-specific-type