[a_humean]

As others have said, there is no single indicator that tells you who is winning a game of dota. Net worth gold difference is usually, but not always, the best default indicator of who has an advantage. Kills and Towers are also a good secondary indicator. However, there are times when all three of those usually reliable indicators aren't relevant.

You can see net worth gold difference by looking at the text that appears in the top center which normally say something like ">1k" for one of the two teams. Its something they recently just added to the spectator UI.

The sliders you mentioned on left usually show individual player net worth, so you can see where resources are being allocated on the team. The "observer" (a camera man I guess) picks which stats to overlay in the top left over the course of the game depending upon which single indicator is most relevant.

In the early game those sliders normally show "last hits/denies" per player, which is about collecting early game resources. In the mid/late game its usually net worth per player. In the late game it starts looking at a thing called "buyback status", which is an ability all players have to spend a lot of their resources to rejoin the game early after getting killed. Using "buyback" is very expensive, scales with networth, and has a 5 minute cooldown.