You can't see the trap effect in Indesign, but you can create a PDF that can. You need to print postscript, not export PDF for this to work. In the print window, turn these settings on: Output > Color: In-Rip Separations and Trapping: Application Built-In.
You need to print postscript, not export PDF for this to work.
I predict you are going to get some resistance to this on the forum. A PostScript workflow has disadvantages over PDF, such as no native transparency in PostScript. Or is this just to view something and discard it instead of using it in production?
Or is this just to view something and discard it instead of using it in production?
With InDesign trapping happens at output, so the only way to preview ID's trap setting is either printing separations or if you want to see them on screen, Jeffery's method. It's probably a waste of time unless you are the printer, because downstream the printer might not even use InDesign's or Acrobat's trapping.
Its just to review it first to make sure the trap is set right. Then will be sent to plates. If i use Post script is this only for viewing? For instance if i set indesign to trap will it show up on the Plate even if I export to PDF? Or is the only way for trap to work period the Post Script way?
For instance if i set indesign to trap will it show up on the Plate even if I export to PDF?
There isn't a trapping option when you export to PDF, the trapping would have to happen when the separations are printed from Acrobat. With Adobe apps trapping is part of the print function—you don't apply traps to the page layout objects.
You could include traps with a composite PDF via In-Rip Separations to Distiller, but you would only do that if you were sure the printer was not capable of handling the trapping, which seems unlikely.
Here's Acrobat's trapping help page