If you know some basic information about the target this is fairly simple.
Search for heat emissions from the planet, matching the expected heat emission of the creature. Remove any readings from things such as volcanoes and factor in the average surrounding temperature.
This method allows you to check inside buildings and underground (to a certain extent - depending on the surrounding temperature and the life form's heat output). As Frostfyre has pointed out, note that this method is completely useless if the environmental temperature is the same as the target's temperature.
Note that due to possible blocking from things in the atmosphere (such as clouds), it's likely that you'll get better results by dropping a drone or a beacon of some sort to get the scans from the immediate area.
Trace movement on the surface of the planet, matching to the average speed of the creature +/- value X based on maximum running speed and walking speed. Eliminate values from any movement of lifeforms smaller/larger than the creature min/max recorded size.
This method helps to reduce the accidental counts of similar life forms - eg: a human may produce the same amount of heat as a dog, but dogs generally move faster than humans. By tracing movement we can subtract the amount of dogs and robots and other anomalies from our total life form count.
Blast the area of the planet with sound waves, record them as they bounce back, and use them to generate a 3D visualization of the area. Use the computer to count.
This helps to remove accidental stacking of heat. For example: if you have people living in buildings and you happen to have 3 people on top of each other, you'll only see the heat signature for the first one. But sonar may allow you to visualize all 3 of them, and as such, apply a correction factor to your count.
Listen for anything like radio waves being broadcast. If you can pick up/detect/intercept any sort of radio communication, you'll at least know "hey, this planet has creatures that have radio/radar already". If there happens to be some sort of (hand)wave in the future that we haven't invented yet that your population knows about, perhaps you can try scanning for those too.
Pattern wise, look for lights on the dark side of the planet. Even from space, we can see our man-made lights from earth. You don't have to look just for lights either, if you can detect noise from certain areas that sound like machinery or if you can detect smoke columns from factories, you've likely found a planet containing sentient life. If you find them, you can perhaps send a drone, or something down to take a look and see if you actually found sentient life.
This is only useful for detecting life above a certain technology level, and it also doesn't give you any information as to population density, species, gender, etc. All it does is tell you if this planet has sentient life capable of creating the type of wave/pattern you're scanning for.
Combine the above methods and you'll have a highly sophisticated, fairly-semi accurate life-sign scanner.
BONUS METHOD: Thank you Twelfth for giving me this idea.
If you don't need a quick result, you can drop nanomachines over the surface of the planet to literally count, obtain images/video of areas, and broadcast said information back to the ship. This would likely give the most accurate result, but would likely take days or weeks for you to obtain any result in the first place. Of course, this assumes that the technology level has already advanced to the level where not you have access to so many nanomachines that you can dump them freely, but your computing systems are also able to process all the information that comes back quickly.