Why do you want to do this?

Well-written HTML will display correctly in any browser, so the correct
answer to this question is to design a template for your output in good
HTML, and make sure your output is correct.

If you insist on a different answer, you can use the HTTP_USER_AGENT
environment variable.  This requires care, and can lead to unexpected
results.   For example, checking for "Mozilla" and serving a frameset
to it ensures that you *also* serve the frameset to early (Non-Frame)
Netscapes, me-too browsers (notably Microsoft[1]) and others who have
chosen to lie to you about their browser.

Note also that not every User Agent is a browser.   Your page may be
read by a user agent you've never heard of, and then displayed by
100 different browsers.   Or retrieved by different browsers from
a cache.   Another reason to write good HTML, and not try to
devise a clever or koool substitute.

[1] At the time of writing, only Netscape 2+ supported frames, and
    some authors considered them koool.  That's changed, but the same
    general principle still holds.