Do you have an HR person? Or an accountant? How do you keep your HR person from being evil and selling everyone's personal information? How do you keep your accountant or finance people from stealing everything the company owns out from underneath you?
For all positions, you should have procedures in place limiting how much damage a person can do. Your default position should be that you trust the people you hire (if you don't trust them, don't hire them or don't keep them), but it's reasonable to have checks and balances.
Even for a small company, you shouldn't have just one "IT person" who is the only one who knows anything. (the same as you shouldn't have just one person who can deal with payroll - what if that person gets sick?). Someone else needs passwords, needs to check the backups, etc.
One thing you can do is to make documentation a priority. Make sure you give the person you hire time to document how things are set up and discuss documentation when you interview candidates - ask what they've done in the past to document their network, ask to see a sample.
It's my habit to always put together a "Systems Guide" that more or less documents everything - what equipment we've got, how it's set up, procedures we follow, etc. etc. It's obviously a constantly-evolving document (series of documents and files in most cases), but at any time you can take a copy and get an idea of how the IT guy has set things up and what critical information someone else needs to know in case the IT guy is hit by a bus. If you really want to be prepared, you could get an outside consultant to go through the systems manual and tell you what they'd need to step in if anything happened to the IT guy.
Or, if you're really paranoid, you could get the outside consultant to come in and compare what's in the systems manual with what they see if they look at your systems. Is there other software installed? Are there extra admin or remote access accounts?