I've noticed a difference in the way Americans and British use 'likely', as an adverb and an adjective (I think) as in 'I will likely visit at the weekend' vs 'It is likely that I will visit at the weekend'. However, in Britain you'll hear 'I will probably visit at the weekend' and even 'I will most likely visit at the weekend'You can tell Richard is not American by the (BrE) at the weekend in his example, but that's stuff for another post. This is the kind of thing that Brits are more likely (ho-ho) to notice because they don't use likely to mean 'probable'/'probably' in ways that Americans couldn't, but Americans use it in a way that sticks out like a sore thumb in Britain.
In the American He'll likely visit this weekend, likely is indeed an adverb. Probably is another adverb that might go there, but for me likely sounds more likely than probably, tautologically enough.
In the American and British She is likely to win the Nobel prize, likely is an adjective. How can I tell? The technical answer is because it's the semantic predicate in this clause, following a copula. The less technical demonstration is to notice that you can't substitute the adverb probably in this case:
*She is probably to win the Nobel Prize.
(Linguists use * before an example to say it's not a possible expression in the language.)
But you can substitute the right kind of adjective (i.e. one that can take an infinitive verb after it):
She is happy to win the Nobel Prize.Adverbs go in adverb places, adjectives go in adjective places. This likely is an adjective.
So far it looks like AmE has likely as an adverb or adjective and BrE has only the adjective. But wait! What's likely doing after most in Richard's other example I will most likely visit...? It's being an adverb in British (and American) English, that's what! As the OED says, the adverb likely is:
Yet another usage that has become extinct in (most of) the UK, but has been preserved in AmE.
Now chiefly most likely, very likely; otherwise rare exc. Sc. dial., or (freq.) N. Amer.
Going back to adjectives, likely also works in both countries as a pre-nominal ('before the noun') adjective, as in a likely reason for her magnetism is her diet of iron filings. But there are certain uses of it that the OED claims as more country-specific. First, this one [earliest examples omitted]:
The most recent example they have of this is from 1883 (but the entry has not been fully updated since the first edition in 1903). I must say, it's not something I'd say.
(Now chiefly U.S.) Of young persons (occas. of animals): Giving promise of success or excellence; promising, hopeful.1793 G. Washington Lett. in Writings (1891) XII. 381, I am very sorry to hear that so likely a young fellow..should addict himself to such courses.1863 Advt. in Dicey Federal St. I. 254 He [a fugitive slave] is..stout and well built; very likely.1883 J. Gilmour Among Mongols xviii. 226 Chinamen go to Mongolia in spring, buy up likely animals.
Next we have:
I can't say I have that one either, though it has some similarity to the fifth sense in the American Heritage Dictionary. The likely spot example sounds fine to me, but I'd put it with sense 3. Better Half doesn't like the example in 5 though (he says it sounds 'very old-fashioned and Enid Blyton'), so maybe it is different from sense 3 and more American.
Of seemly or comely appearance; good-looking, handsome. ? Now U.S. and dial.
On the other side of the Atlantic, we have The Likely Lads. (I'd embedded a YouTube video here, but within hours, embedding had been disabled for that video. So, you'll have to go to YouTube to see it.)1. Possessing or displaying the qualities or characteristics that make something probable: They are likely to become angry with him. See Usage Note at liable.2. Within the realm of credibility; plausible: not a very likely excuse.3. Apparently appropriate or suitable: There were several likely candidates for the job.4. Apt to achieve success or yield a desired outcome; promising: a likely topic for investigation.5. Attractive; pleasant: found a likely spot under a shady tree for the picnic.
According to Wikipedia (the OED is not as clear for this one):
The word "likely" in the show's title is somewhat ambiguous. In some dialects in Northern England it means "likeable", but it may be derived from the phrase the man most likely to, a boxing expression in common use on Tyneside, hence, in Geordie slang, "a likely lad". Another possible meaning is the ambiguous Northern usage of "likely" to mean a small-time troublemaker.
And that's likely all I have to say on the matter. Until you point out in the comments what I've missed.