Either; as long as the cable terminals are clean and making good connections tothe battery. Most put the positive on the cable positive terminal (first) and the negative to some bare metal of the truck (second) so that if a momentary spark occurs it will be physically away from the battery, eliminating any chance of a hydrogen battery explosion. When done, remove the negative lead first. A simple digital voltmeter is available at Sears for under $25, and they frequently go on sale there for $9.99. I have two of theirs.