You can find an overview of the various features supported by the various PCL target platforms under the "Supported Features" portion of this page:
Another option is to use the .NET Portability Analyzer to assess whether your existing library can be converted to a PCL profile.
A third possibility is to browse the contents of the actual Profile that you might use.
Example: Profile 78
For example, you might go here:
C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETPortable\v4.5\Profile\Profile78\
And view all the assemblies within it.
Whichever method you chose, please note that some functionality has to be downloaded via NuGet and the Microsoft BCL library!