As of Dec 2020, there is no certification for Viral Helmets.
Each maker/vendor can choose their own type of filter and there are many types of filters. If you want to be very very sure, I recommend using the filters hospitals use; two types are HMEF or PFT filters. Since these are sold to hospitals, you know you are getting what hospitals get.
Otherwise, you are depending on trusting that the vendor is sourcing a good filter. The bad news is that it's not easy to tell a 80% vs 95% vs 99% vs 99.99% filter without expensive testing (which some universities have done for you). The good news is that filters that are 95% or better are pretty common and offer very good protection. But if you are looking for 99% or 99.99%, that's harder to be sure about.
NIOSH certifies respirators, and that certification costs about $200,000 to get. Some entities (like hospitals and mining) can only use NIOSH certified respirators and filters. The NIOSH certified items are really great. But they are expensive. And not necessarily better. If you really really have to be sure, go with NIOSH certified.
If you have over $1000, go buy a 3M PAPR. PAPR stands for powered air purifying respirator.
They are the gold standard for protecting hospital workers (and miners and those working on asbestos cleanup).
Note: Just like N95 masks, most people say PAPRs should be saved for hospital staff.
Until Viral Helmets are more familiar or start getting certified, you might have to double up: wear a normal mask inside your Viral Helmet. I have heard several reports of TSA agents and airline personnel who won't accept a Viral Helmet and will ask you to wear a mask underneath. Similar things might happen with a school, grocery store, or a city office.
Doubling up is fine. It is less comfortable and you can't see the face. But it's fine.
Doubling up won't necessarily increase your protection. The filters in your Viral Helmet will typically block everything that a mask would block. If your filter is malfunctioning or you need to remove the Viral Helmet, having a mask on will help.