Photos and  text  for checking or replacement the spark plugs  are from the book(Owner's manual Yamaha XV750G and Haynes service & repair manual) .

Spark plugs servicing.
 

A ) Make sure your plug socket is the correct size before attempting to remove the plugs.

B ) Disconnect the spark plug caps from the spark plugs. If available, use compressed air to blow any accumulated débris from around de spark plugs, remove the plugs. (The plugs are in wells in the cylinder heads; use a socket with a rubber insert to grip the plug)

C ) Inspect the electrodes for wear. Both the center and side electrode should have square edges and the side electrode should be of uniform thickness. Look for excessive deposits and evidence of a cracked or chipped insulator around the center electrode. Normally, all spark plugs from the same engine should have the same color on the white porcelain insulator around the center electrode. The ideal color at this point is a medium to light tan color for a motorcycle that is being ridden normally. If one spark plug shows a distincly different color, there could be something wrong with the engine.

D ) Do not attempt ti diagnose such problems yourself. Instead, take the motorcycle to a Yamaha dealer or other qualified mechanic. You should periodically remove and inspect the spark plugs because heat and deposits will cause any spark plug to slowly break down and erode. If electrode erosion becomes excessive, or if carbon and other deposits are excessive, you should replace the spark plug with the specified plug . (Standard spark plug : NGK BPR7ES)
 

E ) If the electrodes are not excessively worn, and if the deposits can be easily removed with a wire brush, the plugs can be regapped and reused( if no cracks or chips are visible in the insulator). If in doubt concerning the condition of the plugs, replace them with new ones, as the expense in minimal.

F ) Cleaning spark plugs by sandblasting is permitted, provided you clean the plugs with a high flash-point solvent afterwards.

G ) Before installing new plugs, make sure they are the correct type and heat range. Check the gap between the electrodes, as they are not preset. For best result, use a wire-type gauge rather than a flat gauge to check the gap. If the gap must be adjusted, bend the side electrode only and be veru careful not to chip or crack the insulator nose. Make sure the washer is in place before installing each plug.

H ) Since  the cylinder head is made of aluminium, which is soft and easily damaged, thread the plugs into heads by hand.

Hints: Since the plugs are recessed, slip a short length of hose over the end of the plug to use as a tool to thread in into place. The hose will grip the plug well enough to turn it, but will start to slip if the plug begins to cross-thread in the hole, this will prevent damaged threads and the accompanying repair cost.

I ) Once the plugs are finger tight, the job can be finished with a socket. If a torque wrench is availeble, tighten the spark plugs to the torque  (12.5Nm) (9 ft-lbs). If you do not have a torque wrench, tighten the plugs finger tight (until the washers bottom on the cylinder head) then use a wrench to tighten them an additional 1/4 to 1/2 turn. regardless of the méthod used, do not overtighten them. reconnect the spark plug caps.

At this web page, spark plugs and what they say? clic here to see.