There's a thing called the 4 and 2 rule. This is usually on your

equity.

It says you multiply your outs times 4 on the Flop and times 2 on the Turn.

So for example. You have a flush draw on the Flop. That gives you 9 cards to complete it. Without looking at a calculator, you have 9 * 4 = 36% to complete it by the end of the hand. If you get to the Turn and you don't have your Flush, now you have 9 * 2 = 18% to complete it on the River. These won't be the exact percentages, but they will be 2-3% away from the real ones so they're good enough. Obviously, making this flush won't mean you win the hand.

This is a very quick way to calculate your odds of completing a given hand. Then you need to compare it with your

pot odds and/or implied odds. That's more complicated and I don't think there's an easy way to do it without experience or calculators.