Unfortunately, in R11, there is no way to determine if a column has a value override, except selecting that column and looking in the override field. You can see this in the formula dependencies tab, too (as mentioned by @Sandor Welfing), but I've found that tab to work only if you're in the time period when the override was set. Otherwise, it will not appear in that tab. R12 might have changed that, but I'm not deep enough into the Cost Transparency changes yet to say one way or the other.