You may have heard of the rule of 4 and 2. That is used to easily and quickly calculate your percentage of hitting your desired hand.
First you need to determine the number of cards available to hit your hand. For example, there are 13 total spades in a standard 52 deck of cards (13 spades + 13 clubs + 13 hearts + 13 diamonds = 52 cards) . If you have 2 spades in your hand and there are 2 spades showing on the flop then there are 9 potential spades left to hit a flush. Don't worry about whether someone else has a spade or one was already mucked. You are only interested in calculating your odds not where all of the spades are at.
Once you know the potential number of cards remaining to hit your hand, you multiple that number by 4 to determine your odds of getting that card on the turn. If you don't hit your card on the turn then you multiple that number by 2.
Keeping with the above example, the rough odds (it's not precise odds) of hitting your spade on the turn is 4 * 9 or 36% chance. If you miss the spade on the turn then your chance of hitting the spade on the river is 2 * 9 or 18% chance.