Count towers, farm, kills, and look at the scoreboard to see which team has the most completed items.

If you are behind play like it.

-Avoid fights where you can
-Let caught out team members die
-Just farm
-Keep as much vision on the map as you safely can
-Buy items that let you wave clear

e.g. Statikk Shiv instead of Phantom Dancer if you don't have a lot of wave clear built into your kit.

Your job when behind is usually to stall and wait until your opponents either make a mistake or until you get get caught up in items and can then take an even fight (though you never want to take an even fight if you can avoid it).

Knowing when you are ahead or behind is a key to getting better at the game. There are things you can look at to tell even though you can't see the actual gold count while in game. Use that information to determine how you play the lane and the game.

;tldr If you are on the wrong side of the level, gold, and/or item mismatch then play accordindly.