But those disks didn't contain a WB.
Did it just boot up to an application, or did the screen flash colours or something (if so, there's a h/w problem somewhere)
So i haven't got a screen for my amiga's yet 
Do you have something called a modulator (I couldn't see one on the pictures). That allows you to plug an Amiga into the back of a TV (not great display, but it should work). Otherwise, you might need to get either:
* An old monitor that plugs straight into the back of the Amiga (new one's won't work unless they're multisync - the Amiga's display is only 15Hz)
* An adapter that plugs into the output and gives you SCART
* A modulator
* A Scandoubler (doubles the refresh rate for newer monitors) or a Flickerfixer (means interlace screens will work well on a modern monitor).
Now i have to find out if a disk contains WB, how can i recognise a WB in a program / game?
When you put in a disk, the display will change. Give it a while and eventually the Amiga should have booted from the disk. If you see an Amiga-DOS screen (a big window with some text and a prompt - probably white text on blue background) then you can do some stuff from there. If you get into a screen with icons, that's Workbench.
and an amiga EXTERNAL (parallel) floppy disk can't be connected to a PC?
Not unless you build you're own controller - there are instructions somewhere on the net for this.