How do I know if it is mereks?

[cmarlowdrive]

Is there a test that the vet can do to find out?

 

[coach723]

I'm pretty certain that all testing is done post-mortem.
There are some other diseases that can show similar symptoms. If you suspect Mareks, I'd recommend you have a necropsy done if you lose the bird, so you know for sure.
There are lots of sites with info on Mareks, here is one: http://www.merckvetmanual.com/mvm/poultry/neoplasms/mareks_disease_in_poultry.html
Best of luck.