Can someone say the bracha upon hearing thunder without having seen the lightning bolt (in any form, i.e. he didn't notice any light at all?) Scientifically, lightning causes the thunder. (Most of the time. I'm not factoring how far the listener is from the bolt.)
So, if someone hears thunder, can he assume that there was lightning, automatically? Or should we reason that since there are sounds similar to thunder, the person needs to be sure that this was actually thunder?