you can't... you need a realy amiga monitor... you will need an extra screen (or two) for the toaster anyway. I suggest you get an old 108x off ebay.
BTW why kind of sync straner are you using?
The only kind the I hear work are the "prevue" ones. But even then the sync strainer is not what you want (won't get rig of flicker and won't workj with most monitors...), you need a toasterscan SCANDOUBLER, wich is impossible to find as they dont make them anymore. the internal DCE scandoubler is cool and $$$ but it WONT work with the toaster anyway..
yeah.....
have you ever ownd and amiga? They are totaly unusual..
I forgot you can also get a 520 wich converst the monitor out put for your tv and then use a regular tv... the 520 are around on ebay for cheap... ($5 -$10) . get ready for a very flickery experience....
stick with it, the amiga is a very cool machine once you get it tricked out, but it's not EASY.