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If you dont have a scope...then you are limited.

Here is one thing you can do, but as FvM and Klaus say, be very very careful.

Dont leave it lying around  with the earth lead cut.

Wear insulating gloves when you touch the VFD (ok i wouldnt bother but i must advise this)

Cut the earth wire, put a  switch on it, so you can switch the earth wire in and out.......then you can see if it still trips if the earth wire is cut.

Earth is though, sometimes needed to  make the vfd immune to noise....so you may do it an alternative way...ie, when you switch the earth out, have a  10n film cap  in parallel with a say 10k resistor across the switch....so there is still a kind of earth connection to the vfd even though you have broken the earth wire connection.

Earth is needed to earth the enclosure in case somehow the electrical insides touch the earthed casing...so be massive careful with this.

Also, you could make up a jig which is just a mains plug with a Y capacitor from L-E and from N-E.....choose say 3n3  for both of them.

Plug this in and see if the RCD trips on a phase where your vfd is tripping.

You could make a couple, and plug 'em all in, and see when the rcd eventually trips.