How do you tell what end is positive on capacitor?

there are a number of different types of capacitors, each with its own identification. on common electrolytic capacitors the negative terminal is shorter and the side with the negative terminal is marked with a white line.

note that ceremic capacitors are bipolar. ie. it doesnt matter which way they are connected. thse are shpaed like a bean and are commonly brown or green.

if in doubt look up the data sheet for the component. just type in the identification on the capacitor into a search engine.