I don't quite understand your question
I was not clear enough, I give you that.
On the 737, you are about to intercept the final approach course, the loc frequency is set, VORLOC is armed.
At what point you can say localizer captured, at what point you are established on the localizer?
1-When the localizer is alive, when you see on your PFD the loc becoming alive, and on your MFD set on APP mode you see the localizer alive (the needle turns magenta)?
2-When VORLOC turns green, vorloc captured?