The canopy is NOT symmetric, but don't worry about it
An accurate center line of the raw canopy does not exist, because the canopy is measurably not quite symmetric, not quite uniform, by a significant amount. At least that's what I found with mine, so don't assume that yours is perfectly symmetric because it probably isn't.
Anyway, an accurate center line of the canopy is not really needed, at least not for the tip-up configuration. This initial "center line" is mostly just useful as an unmoving reference mark for the many iterations of trimming, but it does not determine the ultimate center line of the trimmed canopy.
I initially found the approximate center line essentially as Noah described. That's as good as anything (a laser line would be more straight, but that makes no difference here). But anyhow, ultimately I positioned and trimmed the canopy to find the best fit, which is what we really care about, regardless of this previously marked line.
If I had to do it again, I would forget about this center line exercise altogether. Just take the uncut canopy and place it roughly in position over the fuselage. Move it around to find the most symmetric best possible fit, and then mark your "center line" reference on the canopy to match the center line of the fuselage with the canopy in that position. That's still just a starting point, as you may want to shift things around a bit as you progressively trim the canopy to achieve the best fit. But as a starting point, that's probably more relevant and more useful than trying to measure the canopy's center line in isolation.