Maybe you could use "phent_getvelocity" ?
This function returns a 1 or a 0 on sucess or fail, and the only reason
I can think of for a fail is a NON-physics object.

So, in theory, you could use
if(phent_getvelocity(my_entity, vector(0,0,0), nullvector)) { ... IS physics entity ... }
to test if it is a physics-enabled entity.

(BTW, the "vector(0,0,0)" just dumps the returned velocity into limbo if the function succeeds)