There are two distinct parts to power checks, or as I like to call them power awareness when relating to landings in mountainous terrain.
They are:
1. Awareness of how much power is available to be used (what the engine(s) will provide).
2. Awareness of how much power is in use or for those with a crystal ball, will be used (we can never be 100% sure how much will be used prior to the landing due to all the variables in play).
The difference between 1. and 2, is your power margin. You can think of it as your power safety margin.
One simple way to ascertain how much power you have availble and whether it is enough to land at a given spot is to come to a hover (OGE) beside the proposed landing spot with plenty of air under you. The air below is your escape route should the helicopter not deliver the required power. Of course the result of this exercise could be a false sense of security if the approach to the landing spot is poorly excecuted, for example downwind or into a descending mass of air. In a downwind approach you could easily add another 10% power requirement, which would quickly erode that safety margin that you thought you had. For this reason this type of power check has its uses, but also has its limitations.
There a lots of rules of thumbs around for different helicopter types that are always useful to work out how much power you should need to use once you are in a stationary hover at the spot, but once again they don't help you out if the approach is miscalculated, or poorly chosen. They also all need to be combined with a procedure to check how much power you have available to be truly useful.
In my opinion, the best method to work out if you can land at a spot is to perform a low recce down to a low airspeed (not below translation, approx 20-30 kts depending on machine) as close as you can to your intended approach path. I like to aim for no rate of climb or descent when I am over the spot (will help to identify updrafts or downdrafts), and compare airspeed with groundspeed and power in use. Prior to this, you should have already pulled to the power limit to check that it is available. Sometimes this limit is given by graph, sometimes by progressively adding power till rrpm droop. This method does require having knowledge of how much power you will use at 20-30kts, vs what you will use in IGE or OGE hover. i.e. a bit of experience and a fair amount of practice.
Once you are very familiar with a particular machine and the difference between the power at 20-30 kts vs IGE or OGE hover then, the low recce approach will turn into the final approach without the need to use your escape route and a subsequent final approach.
Some spots on some days do not allow an escape route close enough to be able to make this assessment using this method. In this case, until you have a lot of time and experience in the mountains reading winds and know your machine extremely well, don't go there, choose another spot.
To sum up, firstly, you should have an idea of how much power margin you have available before you go in to a spot, have an escape route, use it if things don't feel or look right, and then after landing assess how much of that power margin you used. Repeat many times until the difference between the first answer and the result are the same 99 times out of a 100. Then it might be time to reduce your safety margin by a small amount and repeat the process over and over again.
Beware though there are a lot of bold pilots who have eroded their safety margins to nothing over the years, then had a bad day and payed dearly for it just because they weren't completely on to it that particular day.