It depends on the type of iron-cored inductor: very broadly, they can be classified into two categories, open and closed.
An example of open type is a solenoid with a cylindrical core (or square section, it doesn't matter provided it is some kind of rod).
In this case, use the formula or calculator for air-core, and double the result (because ~half the length of lines of field are shorted by the core).
The result will be very approximate, like +/-30%, depending on various parameters.
Here is an example of online calculator:
Multi-Layer Coil Inductance Calculator
A closed type is a complete magnetic circuit, with an airgap added: for example, E and I laminations, with a calibrated sheet of insulating material in-between.
In this case, you can use the reluctance formula: L=n²µoA/e
n= number of turns, µo vacuum permeability=4π10E-7, A= core cross section and e= gap length, all in SI units.
This formula should be more accurate, like +/-10%, except if you have extreme gap lengths
multimeters with capacitance and inductance measurement are common these days and relatively cheap
DMM Digital Multimeter with LC Inductance Capacitance and Frequency
DMM Digital Multimeter with LC Inductance Capacitance and Frequency
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