How can I find out what program someone use to create a specific website?

In most cases, you can't. In some cases, if they used Dreamweaver and used some of it's built in scripts and functions, it'll leave behind script code with "MM" in the function names. Frontpage and ExpressionWeb often leave behind tell-tale clues in meta tags.

You can't.

Although FrontPage built sites are usually pretty easy to spot

Occasionally you might find a "generator" meta element in the pages

A better question is, why would you want to?

Beyond that, if others are like me, they use different programs for different things. I have an excel file full of snippets I use. I may use DW to write a table. I may also just hand code something new.

I agree , it doesnt much matter what is used to author a website. Most of the ones I use are a combination of pre-made slightly altered snippets (css, header, footer) and handwritten code.

I use to get teased by my web building friends when I used Yahoo Site Builber to build sites.