How do we determine what it meant to the those who heard the saying?
First, we can look at how similar phrases are used in both the septuagint and the nt. As i said before this is commom idiom to express certainty, especially in the septuagint. See Deu 30:18 for an example.
We have to use the septuagint for usage examples because in nearly every case in the nt 'legw soi' is followed by an 'oti' clause, thus the examples are not parrallel.
We can also go back to sources closer in time to when the NT was written and see how those people interpreted the passage. Below I have quoted a portion of the b-greek discussion on this issue. It shows that putting the comma after today is a valid historical interpretation.
Early Greek manuscripts had no punctuation, but occasionally it is found in some MSS, and this is the case here, where B (the Vatican 1209) has a lower point ((hypostigme) after semeron. Regarding the punctuation used by this MS, it was noted that in general "B has the higher point as a period, the lower point for a shorter pause." (A. T. Robertson, "A Grammar of the Greek New Testament," Nashville, 1934, p. 242) The ink of the uncial letters in codex B was at one time a faded brown color, and in a later century a scribe traced over many of the letters and punctuation marks. However, in Luke 23:43 the ink of the lower point is the same as the letters of the text, and thus it can be traced back to the fourth century C.E.The Vatican 1209 uses punctuation marks also in other places. Thus, at Romans 9:5, ABCL and 26 cursives have a point after sarka. Does anybody know any MS that displays some kind of punctuation in Luke 23:43, beside the Vatican 1209?
The Curetonian Syriac (fifth century C.E.) renders Luke 23:43: "Amen, I say to thee to-day that with me thou shalt be in the Garden of Eden.'"--F. C. Burkitt, "The Curetonian Version of the Four Gospels," Vol. I, Cambridge, 1904.
Below I am quoting from several Greek sources, in transliteration and
providing an English translation. I would appreciate if improvements would be offered for the English renderings.
Tines men houtos anaginoskousin* _Amen lego soi semeron*_ kai hypostizousin* eita epipherousin, hotiet' emou ese e to paradeiso._ ("Some indeed read this way: 'Truly I tell you today,' and put a comma; then they add: 'You will be with me in Paradise.'"--Hesychius of Jerusalem, an ecclessiastical writer who died about 434 C.E. Greek text found in Patrologia Graeca, Vol. 93, columns 432, 1433.
Alloi de ekbiazontai to rhema, stizontes eis to <<Semeron,>> hin' e to
legomenon toiouton* <<Amen ego soi semeron*>> eita to, <<met' emou ese en to paradeiso,>> epipherontes. ("But others press upon the saying, putting a punctuation mark after 'today,' so that it would be said this way: 'Truly Itell you today'; and then they add the expression: 'You will be with me in Paradise.'")--Theophylact, an ecclesiastical writer who died about 1112 C.E. Edition: Patrologia Graeca, Vol. 123, column 1104.
alloi -- to rheton ekbiazontai* legousin gar dein hypostizontas 254:hypostizantas) anaginoskein* amen lego soi semeron*>> eith' houtos epipherein to* met' emou ese etc. ("Others press upon what is spoken; for they say it must read by putting a comma thus: 'Truly I tell you today,' and then adding the expression this way: 'You will be with me' etc.")--Scholia 237, 239, 254. Text found in Novum Testamentum Graece, editio octava critica maior, by C. Tischendorf, Vol. I, Leipzig, 1869,under Luke 23:43.
kai eutys eipen moi hoti amen amen semeron lego soi, met' emou ese en to parad[eiso]. ("And immediately he said to me: 'Most truly today I tell you, You will be with me in Paradise.'")--Descent into Hades, an apocryphal writing of the fourth century C.E. Text found in Novum Testamentum Graece, editio octava critica maior, by C. Tischendorf, Vol. I, Leipzig,869, under Luke 23:43.
ho de eipen auto* semeron lego soi aletheian hina se ekho eis ton
parad[eison] met' emou. ("And he said to him: 'Today I tell you the truth,
that I should have you in Paradise with me.'")--Gospel of Nicodemus (=Acts of Pilate)b287, an apocryphal writing of the fourth or fifth century C.E. Text found in Novum Testamentum Graece, editio octava critica maior, by C. Tischendorf, Vol. I, Leipzig, 1869, under Luke 23:43.
Therefore, at least from the fourth century C.E. until well into the twelfth
century C.E. there were readers who understood the text at Luke 23:43 as "Truly I tell you today, You will be with me in Paradise." On that very day,when Jesus died, he was in Sheol or Hades, and not in Paradise. (Psalms 16:8-11; Acts 2:22-32) He was dead and in the tomb until the third day and was then resurrected as "the firstfruits" of the resurrection. (Acts 10:40;1 Corinthians 15:20; Colossians 1:18 ) Thus, the word "today" at Luke 23:43 does not give the time of the evildoer's being with Jesus in Paradise.