If i got your question right then,
1. why not placing the line only in the bleed area? (this is what i do when ineed marks for folding)
2. when i want to make a line only to mark an area and i dont want it to print (pattern cutting for an example)
i make one color (usually magenta 100%) as spot, and then in the window menu i go to window -> output -> attribute. then i mark overprint stroke/fill (depend on my need)
and mark also nonprinting. in this way you cant determine that it will print only in the bleed area.
As Guytamam says, use tow marks on opposite edges (setting them to overprint in this case should not be necessary, and making them non-printing would be self-defeating). I generally add a slug allowance for marks like this, though, rather than putting them in the bleed, since you never know if the trim is going to be accurate and you don't want them showing.
A word of caution, however, about slug. First, if you are printing yourself, if the sheet size in the printer is not significantly larger than the document size, you probably won't see the bleed area, let alone the slug, so you might want to put some tiny unobrusive mark that you'll have to look hard to find in the actual page area. Second, if you go to print commercially the slug are may never make it past the RIP. I have a brochure I did for years as a 16 x 9 folded to 4 x 9, but when my printer closed his replacement asked me to set up as 15.9375 x 9 so the inside flap wouldn't stick out. I've submitted the project twice to spec, including fold marks, but the folder operator never sees them, apparently, because the RIP crops the page image to the bleed and they are never on the finished sheet. I find this amusing, myself, but my rep does not, and of course I would feel differently if the fold positions were critical or the brochure was inteneded to be more than a throw-away guide map.