You would need to use theCheckOut-Objects webject and set the TYPE to wt.EPM.EPMDocuments. You can use WHERE to locate the document that you want to check out by searching via name, number, or any other unique attribute. You can also just use OBJECT_REF alone (sans TYPE and WHERE) if you know the obid (ufid) of the document.
Look at the WC Adapter Guide, downloadable from PTC support. It outlines how to use all of the different webjects.
In Reply to Bo Li:
I have the same question. Did you resolve this problem?