What is the regularity of appearance of /uː/ and /u/ (or /ʊ/ in RP)? How can I be most sure deducing from spelling alone, that, say, "ooze" is pronounced /uːz/ and "wool" as /wul/? I know that English vowels are peculiar, but I don't want to look up the pronunciation of words in the dictionary that often.
Unfortunately, the occurrence of /u/ vs /ʊ/ is a little arbitrary, in particular because the /u/ vowel occurs in words coming from a number of sources. There are even a few words where either vowel is possible (e.g. "room", "broom", "toothpick"), though /u/ is possibly becoming more predominant in these cases.
However, here are some rules of thumb:
- /ʊ/ is generally only spelt 'u' or 'ou', or 'oo' especially in a few "basic" monosyllabic words ("book", "good", "wood", "wool" but also a few others e.g. "soot")
- if you have some other letter combination ('ui', 'eu', 'ew' etc) you therefore know it can't be /ʊ/ (there are very very occasional exceptions to this, e.g. "Worcester" has /ʊ/ as the first vowel);
- similarly, if you have 'oo' in a "non basic" word, it's probably /u/;
- /ʊ/ isn't ordinarily the final vowel of a word, so e.g. in "who", "do", "woo", "moo" etc the vowel cannot be /ʊ/;
- this extends to declined forms of such words, so e.g. -ed and -ing endings directly after the vowel will generally be an indication that the vowel is /u/ (cf. "wood" vs "wooed");
- though this doesn't affect many words, /ʊ/ doesn't readily occur at the beginning of a word, so in "ooze", "oodle(s)", "ooh!", "oops!" you will generally have /u/, though it's true there is some variation with the 'onomatopoeic exclamations';
- words similar to (because they are derived from) French words tend to have /u/, e.g. "soup", "route" [for UK speakers], "group" etc;
- word-final "-oon" that derives from French "-on" will also generally have /u/ (cf "balloon").
There is also a little idiolectal variation as I mentioned, and in Scotland, the two vowels are often neutralised by some speakers, so e.g. "full"/"fool", "pull"/"pool" are pronounced with the same vowel.
P.S. You can generally assume that /u/ and /u:/ are basically the "same vowel". Like vowels in general, /u/ will be lengthened before a syllable-final voiced consonant, so e.g. in "use" the noun [jus] it will be shorter than in "use" the verb [ju:z]. But this is essentially the same phenomenon as in e.g. "piece" [pis] vs "peas" [pi:z], or "sent" [sɛnt] vs "send" [sɛ:nd] etc.