while most techs will check a DC jack with an ohm meter, checking it the right way will only tell you if it is bad. the wiggle test, with an ohm meter connected will help you out in trouble shooting. Use safety protocols and don't do this if you are not familiar with electricity and its static components, as it could be dangerous. thus said this is what i did.
Everyone knows, that when you get a short in something, that if you wiggle it sometimes the voltage will come back. Well this short may or may not be shorted when you test it with the ohm meter, when it is unplugged. be sure to ground yourself when working with motherboards, chips and components.
So the first thing you can do is tell if the DC jack is loose.
assuming that you cant physically tell there is a problem. (Physical such as loose part on mother board or slighted pin inside DC jack.) Models like the HP 300c have a nice feature with the jack not mounted on the motherboard. this gives more flex to the DC jack, but some people will show you how loose it is, thinking it is mounted on the motherboard, when its design is to be somewhat loose. however it doesn't mean the jack isn't broken.
If you get a Toshiba say MX35 then the jack is mounted on the motherboard and if this is loose sometimes re soldering it to the motherboard will do the trick. However you will still need an ohm meter to verify that the current is flowing correctly through the jack.
On the HP power packs it calls for voltage of 18.5 DC. while this is the correct voltage most of the wall adapters for HP put out slightly more. 18.65-18.75 which is not a problem, you just don't want it lower than the recommended voltage. With the HP DC jack it also has a pigtail on it. this is aprox. 6 inches in length. when using your poles of the ohm meter disconnect this pigtail from the motherboard and check voltage coming through it. You can carefully do this while its connected to the power supply but you will be very close to positive and negative so be careful to not cross or spark them. on the motherboard pigtail you will have 2 red and 2 black you must verify the voltage from each red to each black. this must be the same while it is off the board. (voltage regulators on the MB will take it into the smaller voltages later but at this point they should be the same)
you will now need a friend or clamps. while having the red and black of the ohm meter connected to the pigtail, or touching, you now wiggle the male plug from the wall to the pigtail. if the voltage flexes you may have a short. be certain that the flex didn't come from movement off of the leads to the pigtail part. if it dips in voltage you have found the right part. if it didn't keep looking. the 110 Power supply did you check it to make sure it had the right DC output voltage. is your ohm meter set for say 20 DC (as your looking for 18.5) see if there is a short in the wall plug before the "wall wart or box" is the short in the plug tip?
pigtail end that plugs to motherboard on HP 300c
male jack into the computer
voltage fluxuates when male end is wiggled, male end not shown here....and boy hard to hold and take a photo
this photo below shows the wall jack is putting out correct voltage
and without any flex to the tip of the male jack it would show this
so replacement of the DC jack is now done and back to a working computer.