Here is a quick, easy and informative way to see if you have your https setup correctly on your website. My friendly webhost tech told me about this site, WhyNoPadlock.com
Simply go to www.whynopadlock.com and enter the page on your website address you want to check.
Within seconds it will check and see if your SSL certificate is valid, tell you when it expires and who issued it. Perhaps more informatively, It will also list any unsecure items on the page, such as a link or image not properly posted, (for example if you have hardcoded it and not used the proper OSCommerce format).
If your customers are getting that browser warning that your site contains insecure items, this site will tell you what is causing that.
Another site that provides this same info but a bit more graphically pleasing is www.sslshopper.com