The best things to look at are the gold difference (below the top bar) and the towers remaining (on either side of the top bar). But neither fully captures who's winning. Dota is remarkable for how possible comebacks are, so it's often hard to say with certainty who's winning at a certain moment.

Net Worth Difference is the best single indicator, but there are additional factors like tower difference, item purchasing decisions, hero composition, hero aliveness & buyback status, aegis, etc. Some specific heroes also wildly differ from the norm for each one of these factors. The game is very complex :()

Its not always exactly obvious. But generally speaking, number of kills + towers destroyed is always a good indicator. Another good indicator is overall networth, the team winning usually has 3 out of their 5 players at the top 5 ranks in money value.

One thing you can see is which team is constantly defending and which is constantly attacking. That's a pretty good indicator too.

No guarantees though, games have been known to flop and be unpredictable, mostly due to respawn timers / buybacks lategame. Late game is super critical on who gets first kill generally

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.

A rule of thumb for determining who is winning is the delta net worth: the money icon that appears under the kill counter at the top for the team who's ahead. (as of posting, OpenAI is ahead)

In general its the number of towers they've destroyed + number of kills.

Not always a great indicator but you can tell who's behind.