If the planet is habitable it is probably only habitable within a ring around the terminator where it's neither too cold nor too hot.
From there it should not be difficult to see stars and other planets and be able to tell years and months from their movement.
If the planet has one or more moons they can be used as well to measure timespans from months to days.
If the planet's orbit around the star is excentric enough there might be stronger variations in solar irradiation leading to periodically changing weather patterns, e.g. in the winds crossing the terminator and the overall temperatures, leading to some sort of seasons.
EDIT: About devices used, for the first two points that would just be astronomical devices or simply their own eyes, and in general they could just simply count e.g. the rise of a certain star or planet or moon, or the arrival of a certain warm wind or cold wind. All this could then be the basis for more artificial means of counting, e.g. by taking a moon period and cutting it into shorter timespans of equal lengths by using e.g. sand clocks or water clocks. Once certain shorter timespans are defined, they can again be shortened and with rising technology I can easily imagine mechanical clocks appearing, just like humans did on Earth. Once a basis for measuring timespans is established I don't see why they wouldn't invent the same kinds of devices as humans on Earth did. The main differences just lies in establishing the initial cycles everything else is based upon and those depend on the planetary circumstances.