Hairs on stem (almost all) single (use hand lens or magnifying glass) and patent. Mericarps (don’t have to be ripe at all) densely hairy -----> M. moschata
Hairs on stem (almost all) stellate, with branches patent or more often adpressed. Mericarps more or less glabrous -------> M. alcea
Caution: in M. moschata epicalyx segments tend to be more linear and in M. alcea they tend to be more ovate. However, both species are highly variable in that respect and they overlap to some extent.
Malva pseudolavatera/Pseudolavatera cretica/Malva multiflora, Malva sylvestris and Malva nicaaensis get their fair share of misIDs on this continent too…even has me questioning sometimes if I understand the full variation of each. The Malva parviflora and neglecta IDs are far easier.
It’s universal: keys for Malva tend to be overcomplicated and too much emphasis on features that are of little relevance (Prunus is another such genus). I’m surprised though how common M. alcea seems to be in North America (M. moschata is supposed to be the garden plant, according to the specialists).
A slighlty sharper version. To be honest, Europeans don’t do better.
Hairs on upper parts of stem and on pedicels mostly single (use hand lens or magnifying glass) and patent. Mericarps (don’t have to be ripe at all) densely hairy -----> M. moschata
Hairs on upper parts of stem and on pedicels mostly stellate, with theirs branches patent or (much more often) adpressed. Mericarps more or less glabrous -------> M. alcea
Caution:
Epicalyx : This is always important in identification of Malvaceae and should be checked first. If epicalyx segments are fused then you should look at the (sub)genus Lavatera (often one sees lavatera amongst obs of Malva alcea/moschata). For deciding between moschata and alcea: In M. moschata epicalyx segments tend to be more linear and in M. alcea they tend to be more ovate. However, both species are highly variable in that respect and they overlap to some extent. So, plants with the narrowest of epicalyx segments belong to moschata (or tournefortiana if you are in southern parts of Europe), those with the broadest to alcea, but this takes practice, it’s better to do as above.
Leaves: Leaves are incredibly variable in both species and become even more variable as the seasons progresses (sometimes, one finds plants with almost unlobed leaves). In the first part of the flowering season, Malva moschata usually has deeply divided upper stem leaves with narrow leaf segments and can therefore be easily ID’ed, but this needs practice, so it’s better to do as above.
Colour of the petals: If the plant has white petals, you can safely assume it’s not M. alcea (I don’t pretend that Malva alcea cannot have white petals, but if it exists, it must be very rare).
Some plants are hard to ID, e.g. because stems and pedicels are nearly glabrous etc… According to some people, hybrids do exist, but should then be sterile (mericarps absent or malformed). When in doubt, let the mericarps decide (see key).