nearly right, et:

1) Slow rotation limits coriolis effects and reduces the vorticity (rotational component) of any flows in the core. Vorticity is important for self-exciting convective dynamos, but Venus' rotation should be adequate, according to dynamo theory.

2) High surface temperature does NOT limit heat flow - it simply forces a thinner thermal lid to the convecting mantle (on Earth, we call this the lithosphere and it's about 100 km thick. On Venus it would only be about 30 km). So high surface temperature is not a PRIMARY cause of low core heat flow.

3) But, the lack of plate tectonics may preclude cold slabs descending deep into the mantle and adding strong temperature gradients at the core-mantle boundary

4) And, the history of convective overturn (big overturn about 600-700 Ma), may mean that Venus' mantle is getting hotter right now and is actually almost as hot or hotter than the core (or at least hotter than the adiabatic temperature gradient that compression of a convecting fluid would create). This would suppress core heat flow and hence core convection. so low core heat flow may be SECONDARY result of Venus' surface conditions and geologic history.

5) As to dynamos, whether or not the crust contains magnetic minerals, we would measure a strong planetery magnetic field (if it had a geodynamo), not just from orbiting spacecraft, but by observing interactions with the solar wind and solar storms.

- Flybys and orbital missions to the planets can directly measure the magnetic field, and the sudden boundary between the interplanetary field (essentially derived from the SUn and the solar wind), and the planetary field - i.e. the magnetopause.

- Earth has polar aurorae which are visible from space (or with a telescope, from the Moon or further away). If Venus had a magnetic field, we would see aurorae near its poles, as we have seen for e.g. Jupiter. These would be especially visible when Venus is at closest approach to Earth and presents its night-side.

- Plus, we could perhaps measure the disturbance of the interplanetary magnetic field when Venus passed between the Sun and the Earth.

[Mercury has a geodynamo, but no atmosphere, so no Aurorae. It's magnetic field intensity is only 0.01% that of the Earth, but still detectable - discovered by Mariner 10]

Venus' magnetic field is at least 10 times smaller than this, despite Vhaving over 10 times the mass of M.