Really, the only way to find your Secret ID is through cheating devices.
There used to be a program called Instacheck that 'died' on 12 December, 2013. It could help you find your Secret ID easily. Apparently though, through catching a bunch of Spinda you can actually find your exact SID based on the placement of the spots. I don't know how this works though, so really the only way you can find out now is through using a cheating device. Sorry.
Hope I helped. :)