They can look like a thin dent or a thin grey line..some have slight colour but rare. I had half a pink line twice LOL! Just make sure u read within the time limit, and when time's up DISPOSE of it asap!! Its so hard not to have another glance an hour later LOL!!
U can always post pics if ur camera allows a good pic! I posted a few of my BFNs that looked BFP, im not sure if they are still in TTC section...ill have a quick look...