if you're not actively applying for jobs, that's probably the only measure you can rely on.
getting linkedin messages can imply a few things.
1. the market is short of talents with your skillsets
2. your profile is very attractive to these prospective employers or recruiters
3. recruiters are desperate to close the positions
4. recruiters generally are more active and savvy on linkedin
5. mix of the above

regardless, always a good sign