Natural Product class
It’s very hard to tell for some molecules a priori. For most, there are enough indicators.

Polyketides:


will typically have 1,3-functionalisation patterns, so hydroxy groups or keto groups are typically two carbons away from the next one. And most methyl groups also have a nonmethylated carbon in-between. Note that 1,3 can be expanded to 1,5; 1,7; 1,9; …
if cyclic, will often be only monocyclic or any second cycle will typically be something like a ketal (two oxygens bonded to a single carbon; formed by a hydroxy group attacking a ketone often five carbons away).
if cyclic are typically macrolactones, so large cycles (14, 20 or even more carbon atoms) and linked by an ester group.
can contain phenyl groups or other aromatic systems formed by elimination reactions. Often, these will be 3,5-bishydroxyphenyl groups.
typically contain an even number of carbon atoms in the chain (i.e. excluding any methyl groups mentioned above).


On the other hand, terpenoids:


very commonly have a carbon count that is a multiple of five (even often multiples of ten).
can very often be broken down into isoprene (2-methylbutane) units by paper chemistry.
typically have 1,5-methyl functionalisation (although this can change due to Wagner–Meerwein rearrangements in the biosynthesis).
can have 1,1-dimethylated carbons which polyketides cannot.
often form polycyclic structures composed of small all-carbon cycles (small being 5- to 7-membered rings).
rarely contain lactones or lactams (cyclic esters or amides).
are generally less oxygenated.
very rarely contain more than one or two double bonds, never contain phenyl rings to the best of my knowledge.


With that, you have some criteria at hand to attempt a classification. Artemisinin’s tricyclic structure with an all-carbon cycle looks a lot more like a terpenoid than a polyketide.