How to tell if I am on track for the Genocide ending

The Ruins save points don't give a kill counter, only later ones do.

You need to keep killing more monsters until battles give the message "But nobody came..." This must be done before approaching the boss monster for the area (at the point where you would normally fight them), even if you could normally turn back without fighting them. As long as you're on the Genocide route, save points checked outside the Ruins will give you a count of the number of monsters you have left to kill in the area. The count of monster kills for each section, followed by the name of the area boss, is as follows.

• Ruins: 20 kills (Toriel)
• Snowdin: 16 kills (Papyrus)
• Waterfall: 18 kills (Undyne the Undying)
• Hotland/Core: 40 kills (Mettaton NEO)

You don't need to kill every enemy you encounter, you're allowed to flee from or spare enemies, except for a select few), but you must also kill every unique enemy you encounter. This includes Shyren and Snowdrake. Although the Shyren encounter will happen even if the kill counter is depleted, the Snowdrake encounter will not and can ruin a genocide run.

Note that the requirements is in the number of kills, not battles, so battles with more enemies are more efficient for quickly reaching your goal. You will know that you have enough kills when interacting with a save point simply produces the message "Determination". If you ever leave the Genocide route, there's no way to get back on it without reloading a save from before you left the route.

Source: http://undertale.wikia.com/wiki/Genocide_Route#Requirements

The problem is that the more monsters you kill, the less frequent the random encounters are. This can make the last couple very tedious. However, there is a cool hack/trick that I have found to help you get the last one.

1. Go to an area that monsters definitely can spawn in. They usually don't spawn in puzzle rooms, so just go somewhere that you remember having a random encounter in.

2. If you are in fullscreen, hit F4 to go back to windowed mode.

3. Hold down an arrow key. Any arrow key, doesn't really matter, but don't walk into a wall or an object.

4. While you are still holding down the arrow key, press alt-tab. This will cause the game to lose focus. However, Frisk will continue to walk without actually moving. The game registers this as you walking.

5. Watch youtube until a random encounter happens. This shouldn't take more than 2-3 minutes at most.

This doesn't make grinding very much faster, but it definitely makes it less tedious.