How can we get the toxicity level of a plant without using animals?
Short answer
You can't!
Longer answer

Toxicity is not defined for a plant (or animal), but for specific compounds which are either produced or accumulated in these &quot;toxic organisms&quot;. The concentration of toxic compounds may vary during the vegetation cycle and can depend, particularly in the case of accumulation, on the growth conditions.

Toxicity of particular compounds is typically given against specific endpoints, such as daphnids, rats, etc.

You asked for &quot;the toxicity level&quot;. Which one do you mean?
$\mathrm{LD_{50}}$, $\mathrm{LC_{50}}$, NOAEL/NOAEC, NOEL/NOEC, DNEL,...

If you cannot measure in vivo for whatever reasons, use established QSAR models for specific endpoints.