Only Possible Proof That It's Real
The only possible proof that it's real is the fact that when Quaid is initially struggling at Rekall and the doctor says she hasn't implanted the ego trip yet, Quaid is unconscious. Plus all the moments where Quaid isn't present; like when Cohagen seems genuinely disappointed that he has to kill Quaid (why is that a memory?).
This would imply that it's either real or that you can have memories implanted of what is happening to you while you, the protagonist, are unconscious or not present.