Can you look at a word and see if a "u" should be pronounced as "ooo" or "yoo" by using some set of rules, or do you just have to know the correct pronunciation ahead of time?
For example:
cop[u]late "yoo"
l[u]minate "ooo"
r[u]minate "ooo"
imm[u]ne "yoo"
cons[u]me "ooo"
comm[u]ne "yoo"
I believe the following are reasonable guidelines (although there are numerous exceptions, like cuckoo).
For words that came from Middle English, in standard British English you pronounce long u as "oo" if it follows an "l", "r", "sh", "ch", or "j" sound; and "yoo" otherwise.
In standard American, you can pronounce long u as "oo" if it follows an "l", "r", "sh" "ch", "j", "n", "t", "d", "s", "z" or "th" sound; and "yoo" otherwise.
This phenomenon is called "yod dropping" and exactly what consonants trigger it varies widely with the specific dialect of English (and maybe even the speaker). See Wikipedia.
For foreign words, like kudos, sushi, and puma, all bets are off. All of these were pronounced "oo" in their original languages, but English speakers may (fairly randomly) decide to pronounce some of them with the spelling pronunciation.
Whether u says /oo/ or /yoo/ is determined by whether or not the preceeding consonant is voiced (vibrates the voice box) or unvoiced.
Some guidelines for when ‘u’ says /oo/ or /yoo/ are: It usually says /oo/ when it follows a voiced consonant (g, j, l, y, s, r, z). It usually says /yoo/ when it follows an unvoiced consonant (b, d, p, c, f, h, t).
As languages evolve both in pronunciation and dialect, this 'rule' is weakened somewhat, however it does still hold true in the majority of cases.
Learn a rule and figure out exceptions through exposure is what I always say.
