The "final test" is the ICA facility (military base). You will have to change your disguise if you are going for the other achievements, however in case you dont there are numerous ways to change a disguise. I will name 2 easy ways here:
1) From the start head right, over a fence and than another and you will see a generator. Turn it off and a soldier will investigate. Take him out and change disguises. Pop.
2) Go the same way as in 1) but dont touch the generator. Go a little past it and into the window on the left. Than go into the first door on the right (you will know its the right place when you end up in a change room). On the left side of the talking guards (near another door) there will be a mechanics uniform you can put on. Pop.