What is the standard of proof required to establish adultery for a
divorce? Ideally I like to know about Hong Kong Laws ($20 bonus). But
a good compilation of information base on other countries with similar
laws will still be very helpful.

Here is a very relevant article I found
(http://www.hkreform.gov.hk/reports/rdivorce-e.doc)

Adultery may be defined as "consensual sexual intercourse between a
married person and a person of the opposite sex, not the other spouse,
during the subsistence of the marriage."[1]  The burden of proof
throughout is on the person alleging adultery, as there is a
presumption of innocence.[2]  As divorce proceedings are civil
proceedings the adultery need not be proved beyond reasonable doubt
but by "a preponderance of probability."[3]  Where there is no
confession it may be inferred from established facts (for example, the
content of love-letters,[4] a stay in a hotel room with a third
party[5] or a blood test showing the husband could not be father of
the wife's child[6]).  Evidence of strong passion or inclination on
the part of the persons concerned, combined with evidence of
opportunity, affords strong prima facie evidence of adultery.[7]  Only
one act of adultery is needed for the purposes of the provision.[8]

[1]               Rayden, op cit n 28, at 228.  However in Hong Kong
we must also note the "customary marriage" exception to this contained
in MCO, s 2.

[2]               Rayden, op cit n 28, at 232.

[3]               Ibid, at 233.

[4]               Wong Chan Ying Hon v Wong Chik Wai, SC, Div Jur,
Action No 236 of 1971.

[5]               Blum v Blum (1963) 107 Sol Jo 512 (CA).

[6]               F v F [1968] P 506.

[7]               Rayden, op cit n 28, at 235.

[8]               See Pegg, op cit n 44, at 73-74.