Measure picofarad capacitances accurately
It's quite straightforward to detect a change of capacitance of 0.1pF, as a ratio. The simplest is perhaps to build a relaxation oscillator and measure the frequency and change of frequency digitally, as the test capacitor is connected.

It's very difficult to know exactly how much effective capacitance there is in the rest of the circuit, and any connection jigs, strays, terminals, leads that the ratio is measured with respect to.

The advantage of a relaxation oscillator is that one capacitor terminal is grounded, so the strays are relatively stable. The disadvantage is the strays can be large, quite easily large compared to 5pF.

The alternative is a 3 terminal guarded measurement, which is immune to stray capacitance on either terminal of the capacitor, and only susceptible to strays across it. The third terminal is ground. The method is as follows.

1) Apply a sinusoidal voltage with respect to ground to one terminal of the test capacitor from a known voltage. The strays from this terminal to ground are driven to exactly the same voltage, we are not interested in how much current is required to charge them, the voltage measurement is sufficient.

2) Hold the second terminal at ground, and measure the current required to do that. The most common way to do that is to use a virtual ground op-amp. The strays from the second terminal to ground are held at 0v, so no current flows into them, so the current measurement is accurate.

3) We now know the current through the capacitor for a given voltage across it. Compute the capacitance from impedance and frequency. A capacitive feedback rather than resistive on the virtual ground op-amp allows you to eliminate the frequency from the equation.

Even though the guarded measurement removes the effect of the strays to ground, any strays across the capacitor that are enhanced by your test jig, perhaps a plastic pressure pad that holds an SMD component down onto a footprint, will change the measurement compared to what it would be in circuit without that pad.