You will need to plumb up something which does not leak and either pull a vacuum or pressurize it.... then see how long it holds whichever..
Since it is the connections between the different parts of the overall system which constitute the usual culprits when looking for leaks there is no way to check it until you put it all together... However, I do think it is wise to hook up a ' jumper' across what would be the Rec/Dryer....
Yes, just cap one end of the Evaporator and place a valve and guage on it where you can disconnect from the pump...and be able to read the guage on just the evaporator....
Be sure all the other connections are good... then put the Rec/Dryer in the circuit and close up..and vacuum immediately... that is what the manuals call for... leave the Rec/Dryer sealed until the last possible moment... then uncap and install.... and start vacuuming right then.... so the moisture in the system can't contaminate it...

Did you know that many rental places rent the legit vacuum pumps ? Then you know exactly what it was pulled to.. and how long....
I hate to sound distrustful of mechanics... but.....