I use a multimeter. Plug in the adapter. Then touch one probe to the negative part of the plug (usually the outer casing) and the other to the positive contact (typically a pin inside the plugs outer casing). Your looking for a match on the voltage and amperage listed under “output” on the sticker on the adapter. You don’t need to worry about touching the positive probe to the negative contact and vise versa, it will not hurt it and you’ll get the same reading either way.
If you see voltage or amperage that jumps around from high to low a lot, Ditch that adapter immediately, it can seriously damage anything it’s plugged into.
If the voltageis off by just a bit that’s ok, same goes for amperage. But if it’s more than 1 volt or 1 amp off be concerned.
Note: a laptop should always be matched to an adapter that provides the exact voltage the sticker on the bottom of the laptop calls for. Again, 1 volt or less off is OK but not ideal. Anything under or over can damage the laptop. But amperage can go over and more than what is called for can be desirable as it will charge the battery faster.
To test the jack on the laptop I also use the multimeter. Flip the laptop on its lid so the bottom is facing up. Pull out the battery. Now plug it in (after determining that the adapter is not at fault). Touch the negative probe (this time it matters) to the VGA port or Any other metal port that protrudes out of the plastic casing, what your doing is grounding against it. Now one by one touch the positive probe to each of the pins on the battery terminal. Some carry current but at least two will have none. These are the contacts that carry current out of the battery. The internal voltage and amperage is going to be different from that of the adapter and I can’t tell you the exact numbers because it will vary from laptop to laptop. All I can tell you is its going to be much lower than that of the adapter. Again if you see the readings jumping around a lot that’s bad and could mean the motherboard is fried. If you see no readings then it is probably the jack that is at fault, as power is not getting from it to the battery terminal. To replace jacks, the laptop must be disassembled and the old jack must be desoldered and a new one soldered in to replace it. NEVER USE GLUE.
If it’s what’s called a harness, your in luck. A harness is a jack that has a cable coming off it that simply plugs into the motherboard. They are incredibly easy to replace.