If you will go to the program data folder (hidden) on the system with 3CX installed, there is a sub-folder under 3CX that will have the fax file if it was received. If there, then you know the system got it and can then look at the email side of things. Of course you could always find a friendly to send a fax and have the inform of it success (or not).
The fax server expects to see t.38, so your SIP provider must support this. Some have it, some don't and then some only provide it only on requested lines while others do it for all. The fax server extensions are the only extensions that will use this protocol. If you have analog, then presumably there is no reason to send to 3CX, but rather go to the fax machine directly.