you can't really tell for sure. Usually the joke type sites don't allow it because they want you to come visit their site and in turn view their ads which make them money.
the only real way to tell is to post it, and then clear your cache and refresh the post and see if it shows up or not.
other times they will allow it at first, but then they might notice in their logs after a while that that pic is getting downloaded huge number of times, and then either delete the pic or replace it with something else.
I know in the past when I've run other websites and noticed people hotlinking my images I'll replace the pic with something nasty along the lines of the Most Discusting pic thread so people will all of a sudden get some nasty tubgirl pic on their site instead of the image they had origianlly wanted. That'll teach those bastards for stealing my bandwidth
thats why I personally avoid hotlinking. If its not something that needs to be on the fibc server then I'll just throw it on my shaw webspace (where I know it is allowed) and hotlink it from there.