Things to remember
There are four companies' trucks in the map and six trucks. This means there are two companies that have many trucks at the scene. The correct truck is labelled with a company only has one truck there.
- Omni Exports
There are two ways to figure out the right truck.
If you can see the trucks from the spawn point, rule out the companies that have two trucks immediately. Then check the colors of the remaining trucks and rule out those who have a pair of the same color. If you're left with no trucks, you need to check the computer (option 2) for the company that has the voting machines. If you're left with one truck, it's a pretty safe bet it's going to be that one. If you're left with two trucks, check which one has an open container matching its color and rule it out (option 1).
If you can't see the trucks from the spawn point, it might be easier going straight for one of the options below, but you could sneak to the trucks and follow the instructions above.
Option 1: Containers
As Bain hints, you can figure out the correct truck based on the open containers in the map. Find 3 open containers, which are not white, check if any of the open ones contain blue and red "Your Vote" -posters. (You can see this from quite far away as they're large.) If you find the posters, that container's color is also the truck's color. If you don't find the posters, you can dismiss those colors.
Option 2: Computer
Find the computer in one of the rooms in the two warehouses. You're not trying to figure out the cargo from the screen. It's irrelevant. There are only three companies listed on the computer. Go through them all and remember their names (remembering the first letter is enough). The correct truck is from the company that was not on the computer.
Using the computer seems like a bulletproof way to do this while the container colors seem like they have a small chance to fail (2-3 possible trucks after checking the containers), but spotting the containers is usually easier than sneaking to the computer, so it's a tradeoff.