The cheapest option is to remove the boot, and see if there is any spark from the end of the lead to earth. Should jump 5-6 mm

I would stick a small nail inside the wire and leave about 1/4 in gap to the cylinder head and kick it over or you could grab the nail and see if you get shocked Just kidding 

There are new ones available.
Also this is a very good tool to determine spark.

www.rcpw.com/trillium-spc-7200-spark-che...wKwV72caAvGIEALw_wcB
 

There’s a chance it’s not the spark plug cap. The DT3 might have a female bullet connector that is supposed to be an open circuit because it’s a test lead. I know the DT2 and DT1 has a black lead that will ground out spark immediately if anything is connected to it. My best recollection is that female black wire connector resides under the oil tank by the 45 degree frame rail going to the left rear shock. Many people have connected the left rear turn signal there in error.

asco

Cheapest thing make sure none of the swtiches are off and "clean your points". 

No need to buy an old original boot...  Yamaha still sells them new.

Probably just an NGK which you can buy pretty cheap from a lot of places.  Are you using the original non-resistor plugs?  If not or if you want to start using the more common resistor plugs you can buy an non-resistor NGK cap. 

With that magic spark checker tool, would it still read if the plug was shorted out--dirty--baked on oil shorted where the plug tracks to earth down the dirty porcelain. I mean-- if there's life in the plug lead but she ain't gonna fire up regardless.

here is a question regarding the resistor plug and the resistor plug cap. 

can you use the resistor plug and resistor cap or is this a problem waiting to happen?

PFC it can and probably will effect your spark to much resistance it will run but not  great
  Trailboss did you get your B8es from a known source there are fakes out there 

B8ES seems to be out of production so harder to find.

LZFH is the NGK plug boot number if you want to swap to BR8ES resistor plug.

LB05F is the NGK original type 5k ohm resistor cap if you want to stay with the B8ES non-resistor plug.

Either one can be found on ebay for less than $15 including shipping. 

Yes NGK are being faked you are good getting them from the dealership