Where can I find how much lifesteal I have in League of Legends for my champion during a game in the HUD?

By holding C (or clicking on the icons on the left side of your champion portrait) you gain access to the champion stat panels. The panel on the top right includes the most relevant numbers (AD, AP, HP, etc.) One of those numbers is Life Steal. Hovering over the icons has a legend pop up identifying each icon.

In-game, by holding C in your KeyBoard it will show a panel with the personal champion stats. The panel shows, by default, on the left side of the icon of your champion, as you can see in the image below. Each icon has its own tooltip that will display what stat you are previewing.

The LifeSteal icon is the one highlighted as blue.