Windows: hold Alt and type 236 on the numeric keypad
Mac: press Option + 5
iPhone/iPad: press and hold the number 8 key, then choose ∞
Android: press and hold the 8 key, then choose ∞ if available
Copy and paste this symbol: ∞
In Microsoft Word: type 221E, then press Alt + X
In HTML: use ∞ or ∞
In LaTeX: use infty