Is it possible to know the total hashpower backing a currency?
My goal is to know how much hashpower is backing each currency. I don't know if it's possible, if so, how?
Generally cryptocurrency network mining difficulty is dynamically adjusted by the current available hashing power. Thus, the hashing power must be known. This information is usually disseminated on raw protocol level and through different blockchain explorer web sites.
There are really two different measurements you have to be concerned with for a particular blockchain:
The current difficulty is a function of the current target. This means that when the target is lower (more difficult), more network hashing power is required to produce a single block. When comparing two different blockchains, this value can be used to determine the current investment being made into mining that chain.
The total chainwork, on the other hand, is a measurement of how much work has gone into the chain over time. For example, if blockchain A currently has a higher difficulty than blockchain B, but historically blockchain B has had a higher difficulty, then blockchain B will have the higher total chainwork. This will be true until A has been more difficult than B for long enough to overtake the total chainwork of B.
In the case of a forked blockchain, they both have the same chainwork up until the point of the fork. From then on, the chainwork is determined by how much hashing power has gone into each branch of the fork.