MOSFETs

==Inwerp==
Mosfets usually fail with a short circuit
It happens because gate is actually isolated from semi-conductor npn doped sandwich
As soon as it has small leak, it exponentially grows up at some point
And you have full short between gate drain and source - insulator simply is not here anymore. ESD damage is a real thing since higher voltages might make a small "hole" in the insulator and mosfet will smoke as soon as running current reaches enough rage to "find" this little hole. It might happen after you give device back to customer and the only way to avoid it - do not ignore ESD safety at least when you're working with MosFETs directly. 
On p mosfets you have basically the same situation except you are supossed to have continuity between drain and source if gate is low but not between these two and gate, since it is always isolated on FET(edited)
P mosfets sometimes fail with no short to gate, it could be diagnosed off-circuit
Probing techniqueIt is always a good idea to test every new sanded and probably fake MosFET before you solder it on board. Otherwise you risk further damaging the board instead of repairing. 
It is actually quite easy. Lets start with one simple, N-Channel 820-00165 CPU VCORE SiSA18DN N-Channel MosFET (In most cases EVEN numbered MosFET are N-Channel, Odds are P-Channel, but you rather double-check) 
Step 0: short MosFET's Gate to Source. It might be charged from your hand or soldering iron. Gate acts as a capacitor, so you need to discharge it before testing. 
Step 1: Set multimeter in diode/ohm continuity mode and Put BLACK probe on Drain (bigger plate), RED probe on Source. No continuity means MosFET is not shorted, in diode mode you will also see something like 500mv voltage drop of the internal diode. This is fine. 
This is not a real diode but part of "parasitic" NPN transistor which is a part of MosFET construction itself. This is also a shortcut to remember difference between N and P channel mosfet representation in schematics. NPN transistor could be represented as two diodes with cathodes ( - ) poiting outwards. And this is exactly what you see when you look at the S-Node of the N-Channel MosFET: two diodes, pointing to Drain and Gate respectively. On P-Channel MosFET, both diodes will be pointing to S which represents PNP transistor as well. So if you remember polarity of normal diode, you can use it to read if its N or P channel MosFET, simply see the "arrow" as a diode which indicates if it is P (anode) or N(Cathode) on Gate. 
Step 2: Put the RED probe on Gate to charge it. Now go back to step 1, put RED back to SOURCE. MosFET is now conductive, you hear a beep. 
Step 3: put RED probe on Drain, BLACK probe on Gate. This will discharge the Gate and now if you put BLACK probe on SOURCE there will be no continuity. 
Testing P-Channel mosfets is basically the same process but you need to switch Red/Black probes. Use probes as a low-voltage battery which is used to apply negative voltage on gate. 
Brief explanation: Multimeter uses low voltage in Ohm-continuity mode. It is enough to charge MosFET's Gate but it makes sense only off-circuit since Gate might be quickly discharged by power IC. Diode mode will also detect internal diode since it uses slightly higher voltage to measure. 
Important info: Many amateurs check MosFETs by simply probing some random points thus getting random results. If you simply put some probe on Drain and go with some other probe through other pins, you may accidentally charge the gate and probably get a false "shorted mosfet" result. It is also possible that perfectly fine MosFET in circuit will also beep simply because its gate is still charged. 
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==piernov==
Basically MOSFETs have a diode allowing current to flow from drain to source even when gate isn't triggered.
But as it is a diode ,it has a threshold voltage/forward voltage drop.
So when MOSFET isn't turned on and current is flowing from drain to source, you'll measure a voltage on the source lower (by like 0.3-0.7V) than on the drain.
(in the case of the P-channel, since the context was about DC-in MOSFETs on Macbook boards that are P-channel)
But basically current flowing through body diode is an unwanted effect. The forward voltage drop means you waste power proportional to the current that needs to be dissipated (P = Vf * I). So you want to turn on the MOSFET anyway if you want current flowing through it.
Meaning that in general it's not normal to measure a voltage drop similar to a diode forward voltage drop across a MOSFET.
In the case of DC-in MOSFETs, what can happen is that the MOSFET that normally blocks the current shorts across drain/source/gate. This short causes the gate to be at the same voltage as drain/source, and it usually also causes the gate for the other MOSFET to have the same voltage, so it's turned off, either because they're tied together or the charger IC tries to turn them off. So current reaches the reverse polarity protection MOSFET, which would normally block the current from flowing in the other direction, so it flows through it but it's not turned on, so you measures a voltage drop after the DC-in MOSFETs compared to before them which is not normal.
The other failure mode of the DC-in MOSFETs is the reverse polarity protection MOSFET shorts across drain/source/gate, gate has the same voltage as source/drain, it's tied to the other MOSFET, the one that blocks current flowing in the correct direction, so it's turned off and never lets the current flow.
What's confusing for beginners in this situation is that the MOSFET that's bad is the one that lets the current flow, not the one that is blocking the current. Every beginner will jump on replacing the one that blocks the current and obviously it'll not solve the problem. Then they Sorin it and they don't understand why they have 19V on the main power rail but it's still not turning on, or it's turning on but the battery is not charging, or the laptop is throttling or whatever weird stuff happens.
This second situation is much more common on non-Apple laptops.