It's a scam.
The real Multiplan, Inc. has a disclaimer on their site. The
check will not be honored.
However, if you've any doubt, feel free to go to your local bank
and ask them. There's no harm in that, and it's not a crime to
ask.
Whatever you do, though, do not send any money or information to
them, even if they tell you that they've a bigger check for you, or
that they'll "activate" the first check.