Checking user-agent generally does work pretty well, as does robots.txt.

If you actually need to identify (rogue) bots which use browser UA strings and ignore robots.txt, your best chance would be to look at the patterns in the timestamps for when pages are requested:

• Humans will generally either open one page at a time, making single requests at wildly irregular intervals (possibly interspersed with HEAD requests when they use the "back" button), or open everything in tabs, producing flurries of several requests within a few seconds followed by generally longer intervals of few-or-no requests.
• Bots will tend to request pages at a relatively steady rate - even if they have randomness in their delay, it's rarely more than half the base interval - and often quicker than a human would.

Don't rely on javascript to make your determination. Some of us use the noscript plugin, which blocks javascript from running unless it comes from a whitelisted site, but we're still not bots.

Anyhow, though, what are you attempting to accomplish by identifying what's a bot and what isn't?

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