Rats are generally nocturnal animals. So you might see signs of rat infestation before you actually see any rats. There are obvious things to look for, such as rat droppings, which share the general dimensions of a sultana. Also, chewed bits of furniture or wiring could also be a sign of a rat infestation, although that could also be an indication of mice.
Holes in the wall are another sign of rat infestation, as are smears
on the wall. These come from the oil on the rats' fur, which comes off when they rub against it. Rat infestation is serious due to the danger of contracting diseases.
When you'll come face to face with a rat, it won't be a pretty sight. You should be careful that you don't get into a position where they can bite you, as this will be very painful and quite serious.