I want a sure way to know this ...person....did what they are telling their client they did.
Perhaps someone has more creativity then myself but I do not know any means by which you can be 100% certain a sitewide 301 is implemented without seeing the file on the server. The "file" varies based on the server type. As you know, for Apache servers the .htaccess file is the right one.
Even if you saw the .htaccess file, it is possible for another file to overwrite the command. The way I always have verified is by looking at the site itself. Check the home page and a few other pages. If they are all 301'd properly, then I presume the developer performed their job correctly. It would actually be a lot more work for the developer to attempt to fool you by 301'ing part of the site but not all.
I also suggest ensuring your site's www or non-www standard appears correctly in your crawl report.
Is my assumption that if a 301 was done in .htaccess, there should be no www showing in Google Site:?
That is not necessarily true. If you have a site which shows mixed URL results, then overtime the results from a site: search will be standardized, but it will take time as Google needs to crawl each and every page of the site and see the 301. Also if any page is blocked by robots.txt for example, then Google may not see the 301 for that page and still list the old url.
If you changed the Google WMT preferred domain setting, then it is true you will only see one version of the URL. I would specifically advise you NOT to change that setting in this case as it may cover up the developer's issue which you are trying to locate. As for now, you can wait 30 days and perform a site: search. Investigate any bad URLs you find.