How do I figure out which ligand will form Low spin and which High spin without using any magnetic data like whether the complex is paramagnetic or not. Is there any classification of ligands that form high spin and low spin?

Ligands that produce a small Δ are called weak-field ligands and lie at the left end of the series. Ligands that produce a large Δ are called strong-field ligands and lie at the right end of the series. For example in case of octahedral complexes, the weak-field ligands give rise to small Δ_o and thus are high spin while strong-field ligands give rise to large Δ_o and are low spin.

Water always gives rise to small splittings of the enregy of the d-orbitals for first row transition metal ions and hence is referred to as a weak field ligand. Conversely, CN^- is a strong field ligand, since it causes large splittings of the energy of the d-orbitals.