Different style guides recommend different approaches to hyphenating prefixes, but most sensible ones start from the proposition that the decision to hyphenate or not to hyphenate should be based on the readability and sense of the resulting word. Unfortunately, attempts to spell out a viable general rule entail spelling out multiple exceptions, as we see in the way Chicago Manual of Style, fifteenth edition (2003) handles the issue:
Words Formed with Prefixes
Compounds formed with prefixes are normally closed [that is, not hyphenated], whether they are nouns, verbs, adjectives, or adverbs.
A hyphen should appear, however, (1) before a capitalized word or a numeral, such as sub-Saharan, pre-1950; (2) before a compound term, such as non-self-sustaining; pre–Vietnam War [clarification about en dash use omitted]; (3) to separate two i's, two a's, and other combintions of letters or syllables that might cause misreading, such as anti-intellectual, extra-alkaline, pro-life; (4) to separate the repeated terms in a double prefix, such as sub-subentry; (5) when a prefix or combining form stands alone, such as over- and underused, macro- and microeconomics.
Even those five exceptions don't cover all of the special cases, though. Another important exception arises when omitting the hyphen yields one sense of a word and including it yields another, such as resign (withdraw from office) and re-sign (sign again).
Anyway, in its list of words formed with various prefixes, Chicago offers this entry for re:
re: reedit, reunify, reproposition, but re-cover, re-creation (as distinct from recover, recreation)
It seems clear (to me) that Chicago would endorse reoffend over re-offend.
That's just one style guide, however. Others might arbitrarily adopt the rule that hyphens should by used whenever a prefix that ends in a vowel attaches to a root word that begins with a vowel (for example, re-enter, re-order, re-acquaint); but in that case, the guide would probably find it necessary to carve out exceptions for very familiar words that almost everyone spells without a hyphen (such as rearrange, reinstall, reopen, and reuse).
Assuming that you aren't required to abide by the dictates of a particular style guide, I think that you can't go far wrong if you simply decide for yourself whether in a particular instance including a hyphen after a prefix improves clarity, makes no difference, or makes matters worse (as in using re-form when you actually mean reform).