@James *********
as a rule you have to show onward travel OUT of thailand about 60 days after your first entry to get that visa (at least using the online eVisa system you should show WHY you need an METV)
Now you won't be asked for anything once you land here at passport control because you're entering with a valid visa.
Go to the website of the thai consulate you're applying at and read the requirements for the METV or email/call them and talk to them about it