There are all sorts of shell extension registration rules. You can look each type up manually, following the registry path from HKCR\.ext to the ultimate handler in HKCR\CLSID registry key, but why waste your time fumbling with Wow6432Node and whatnot? I wrote a tool to make everybody's life easier. It can check the registration of:
To use it, open a console window and type the tool name followed by the file type you want to examine, e.g. for PDF you would type:
If you find there's something missing, you must install a namespace extension that will fill the gap. Sometimes you are ok in 64 bit but missing out on 32 bit plugins (or vice versa) — this tool will tell you if that's the case. That's all folks!
You may have heard of a similar tool called Shell Extension View. That's a different kind; it shows all the shell extensions on your system. My tool will query per file type.