Question

How to find dimensional formula of latent heat?

Answer 1

ΔQ = mL
where ΔQ is the heat 
           m is the mass of the body
           L is the latent heat ( of fusion/vaporisation)
heat is energy and energy has the dimensional formula $[M L^{2} T^{-2} ]$
LHS and RHS should have the same dimensions.So the dimensional formula of latent heat is $[ L^{2} T^{-2} ]$

It is better to remember the dimensional formula of energy as it will come in handy in a lot of problems. Otherwise you can derive it using E = W = F.s 
and F = ma.