If he did use them, when did he stop using them? How much did it really affect his career? Let's take a look at his stats since 2003, and look at the differences, since he most likely wasn't using steroids every year.
2003- .296 average, 181 hits, 47 home runs, 118 RBI's
2004- .286 average, 172 hits, 36 home runs, 106 RBI's
2005- .321 average, 194 hits, 48 home runs, 130 RBI's
2006- .290 average, 166 hits, 35 home runs, 125 RBI's
2007- .314 average, 183 hits, 54 home runs, 156 RBI's
2008- .302 average, 154 hits, 35 home runs, 103 RBI's
So looking at his stats, how much was he really affected by steroids? They look pretty similar year-by-year, and if someone was to take a guess of when he took them, they would most likely say 2007. But it seems impossible that he could have taken them in 2007, with all of the testing going on. It seems that every other year, his power numbers tend to go down, but that would say that he's taking steroids every two years, which would make things crazy. So it is plausible to say that if he took the one time, he continued to take them. Or that he stopped after doing it once.