Experimentally, the power taken by the friction between the engine's components ( P_{f} ) is found by substracting the measured power at the crankshaft (P) from the potential power that we can obtained from the measured gas pressure acting inside the cylinder(s) (P_{i}).

(1) 
Dividing equation (1) by the measured volumetric flow rate (Q_{m}), we get an equation of pressure:

(2) 
Where:
FMEP_{d}  = friction mean effective pressure differential 
IMEP_{d}  = indicated mean effective pressure differential 
BMEP_{d}  = brake mean effective pressure differential 
So, the friction mean effective pressure represents the equivalent pressure «taken away» by the internal losses.
For reciprocating engines, we usually divide equation (1) by the theoretical volumetric flow rate, such that we obtain the average values instead of the differential values (see the BMEP page for more info).
Estimation
A relationship has been well established between the mean piston speed ( v_{mps} ) and the friction mean effective pressure to make a good estimation. This site goes further by an attempt to relate it as well to the indicated mean effective pressure:

(3) 
Where K_{sum} is the sum of all values (K_{x}) for the engine components in use, as presented in the next table:
engine component  K_{x} 
camshaft  0.20 
lifter  0.20 
rocker  0.10 
crankshaft  0.20 
oil pump  0.10 
water pump  0.10 
fuel pump  0.05 
distributor  0.05 
Main source: Chulalongkorn University, 2103471 Internal Combustion Engine, Lecture 13 notes, p.11. (original link)
Wankel engine
To relate the friction losses of the rotor of a Wankel engine to the piston of a reciprocating engine, as a trial, this site uses the mean rotor tip speed such that FMEP_{d} becomes:

(3a) 
Remember that Wankel engine have no camshaft, lifters or rockers.