Okay, here goes:
- Calculate the coefficients $\langle c_j : j\in d\rangle$ of the degree d-1 polynomial f'.
- Run quantifier elimination over real closed fields on the formulas
$((\displaystyle\bigwedge_{j\in d} ((a \leq x_j) \land (x_j \leq b))) \implies$ $(((((\displaystyle\sum_{j\in n} 1)\cdot (\displaystyle\sum_{j\in d} (c_j \cdot (\displaystyle\prod_{j\in d} x_j))))+1) \leq 0) \lor (1 \leq ((\displaystyle\sum_{j\in n} 1)\cdot (\displaystyle\sum_{j\in d} (c_j \cdot (\displaystyle\prod_{j\in d} x_j))))))$
until the result is $ \; (0 = 0) \; $. - $\frac1n$ is now a lower bound for $|f'|$ in the interval $[a,b]$.