How to estimate the magnetizing inductance in transformer?
Here's a commonly used model of a real transformer:



Rp: Primary winding resistance

Lik: Primary leakage inductance

Rc: Core losses

Lm: Magnetizing inductance

According to the model above, measurements while secondary is open provide info about Lm. 

Actually, the primary inductance measured with a LCR meter at a single frequency (generally 1kHz or 120Hz) while the secondary is open is an approximation of magnetizing inductance. However, Lm varies with core properties, temperature, frequency etc. So, measuring primary impedance for wide range of frequencies is the best practice to estimate the magnetizing inductance. 

Procedure: First, you need to know the frequency that the transformer is designed for (\$f_{tr}\$). Then, apply an acceptable level of voltage with different frequencies (e.g. if ftr = 100kHz then the range can be [1kHz - 1MHz] with 50kHz or 100kHz steps) and measure the impedance (\$Z_p\$). If you put the values in a Z-f graph in logarithmic scale (both axis) then you'll get a linear view where you can fairly estimate Lm from \$Z_p = 2 \pi \ f \ L_m\$.