It depends on the individual sensor, but the main issue will be range. Many sensors are capable of operating from orbit (though the height of the orbit will depend on the planet’s mass and the ship’s velocity, it will be a relatively small distance for most sensor types), but may need a large receptor dish to do so effectively. Pretty much any type of sensor can be used on a planet from orbit, but you may have trouble with a smaller ship.
Life form indicators (LFIs) are not actually sensors in and of themselves, but computer programs that take data from other sensors and interpret it to come to a conclusion. A full-spectrum transceiver is used, because they are designed to detect all types of objects, energy, and fields. When interpreted by the LFI, they will return such information as temperature, motion, mass, and the life support settings of a ship (including atmosphere and gravity).
Because full-spectrum transceivers are so broad, they aren’t very sensitive, and a large receptor dish is needed for long-range scans. The LFI is only as good as the sensors, so a ship without room for a large dish will have trouble getting an accurate reading from a planet. (Additionally, I do not know whether an LFI would be able to distinguish between beings on a planet. I can’t think of an example of someone scanning a planet for species or population. However, the full-spectrum transceivers would be able to detect large concentrations of beings by their heat signatures and, depending, may not need to be totally accurate, so if this is useful information to have then you probably could get it from the sensors of a small reconnaissance ship.)
Other types of sensors that could be used on a planet include: electro-photo receptors, which combine visible light, ultraviolet, and infrared to form a hologram (only useful at short ranges); dedicated energy receptors, which detect electromagnetic emissions, including comlink transmissions, navigational beacons, heat, and laser light (these are what are used to detect enemy communications); and crystal gravfield traps, which detect gravitational fluctuations such as the presence of a ship or a planet. All these sensors could conceivably be installed on a small ship, though CGTs are rare and expensive.
Hope that helps!
~ Jacen