Is it possible to know the total hashpower backing a currency? 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?
Yes.

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.

Hashing powers of different Ethereum forks.