How strong is the electrical field inside a microwave oven?
I will try.  The power level of the microwave in Watts will tell you what it might transfer to your cup of coffee.  The energy density will be \$\frac{e_0\cdot E^2}{2}\$ , where \$e_0\$ is the electric constant, 8.854187817E-12 Farad/meter and the electric field, E, is in volts/meter. 

$$\text{EnergyDensity }= \frac{e_0 \cdot E^2}{2}$$

If the microwave is empty, I think that most of the power used is heating the microwave generator, and there will be some absorption in the walls, glass plate, moisture in the air, door, etc.  If there is water, then you might take a look at the dielectric loss chart (blue) in this description of electromagnetic absorption spectrum of water.  https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water

I thought that the power should be low when the microwave is empty, since it is not doing any work if there is nothing to heat, but the watt meter says 1267 watts whether it is empty or full.  I would have it transmit energy and only heat the water.  Seems kind of wasteful to me.

But to answer your question, it is easy enough to google [electric field inside microwave oven] to find http://physicsed.buffalostate.edu/pubs/TPT/TPTMay02Microwave/TPTMay02MicrowaveOrig.pdf where he says 2000 volts/meter and 2.8 megawatts/meter2.

Wikipedia's article on Intensity can give you the Intensity in Watts/m2 and its relation to the electric field.  https://en.wikipedia.org/wiki/Intensity_(physics)

$$I = \frac{c \cdot n \cdot e_0 \cdot E^2}{2}$$

in Watts/m2.

$$Intensity = SpeedOfLight \cdot IndexOfRefraction \cdot EnergyDensity$$

The link to the refractive index gives 1.333 for the refractive index for water.  \$e_0\$ is also called the "vacuum permittivity" as above.  Plugging in 2000 Volts/meter gives 7.07668E3 watts/m2.  To get his 2.8E6 Watts/m2 would require 39.783 kilovolts/meter2.  So something is not quite right.

Googling [power absorbed in a microwave] leads to https://www.emu.dk/sites/default/files/physics_of_microwave_oven.pdf where that blue curve in the Wiki article is explained.  It is the frequency dependent dielectric constant of water (that is not the proper name, but the easiest way to remember it).  To get his 2.8E6 watts/m2 would require the refractive index to be about 53.  Since that does not seem to be the case, keep going.

I will leave you to read his equation (8) which gives the power absorbed as follows:

$$P = 2\pi f e_0 e_2 E^2 VolumeOfObject$$
$$Power = 4 \pi \cdot Frequency \cdot e_2 \cdot EnergyDensity \cdot VolumeOfObject$$

\$e_2\$ is the blue curve.  It is the absorption part of the dielectric constant.  The blue curve that is somewhere between 20 and 40 depending on the temperature.

Omega (&omega;) is 2&times;&pi;&times;frequency.  Radians per second mean nothing to me here, so I use frequency. \$e_0\$ is still the electric constant, or electric permittivity.  The volume has to be in meter3.  After 50 years I learned to always do the calcuations in SI units, regardless of the convenience when working in some small domain.  E is the electric field in volts per meter and the power is in Watts.

Not one to give up easily, I googled [power absorbed in a microwave "electric field"] and found http://www.pueschner.com/en/microwave-technology/basic-calculations.  He uses the same expression for the power, but gives his slant on the problem.  He says the \$e_2\$ is about 12 at 20 Celsius. And he did not multiply by the volume, but calculated the power density (watts per meter3, watts per cubic meter).  

One could simply take the effective wattage of the microwave (my 1267 watt microwave only produces 900 watts effective), and divide by the volume of the oven (30&times;30&times;20 cm for mine is 0.018 m3) to get 50,000 watts/m3.  I think that will lead somewhere useful, but it is time for lunch.  I hope I have left some useful clues.

These kinds of problems are much better handled in a spreadsheet or calculator, rather than text formulas.  I normally take the energy density (Joules/m3), multiply by twice the magnetic permeability (1.25664E-6 Newtons/Ampere2) and take the square root to get the the magnetic field in Tesla.  Then multiply that by the speed of light or gravity to get the electric field in volts per meter. Or take the energy density (Joules/m3), multiply times the speed of light and divide by 4 to get the intensity in Watts/m2.

$$EnergyDensity = \frac{e_0 E^2}{2}$$
$$Intensity = EnergyDensity\frac{SpeedOfLight}{4}$$

in Watts/m2.

I use energy density to convert gravitational energy density to electromagnetic units.  Then the 2000 volts/meter would be equivalent to an acceleration field of 172.34 nanometers/second2.  And its effect would depend on the absorption.