There is free landlord software out there that allows you to customize your online applications and ask for documentation (proof of income/proof of employment etc).

In addition, verify income, employment, and call landlord references.For leases, just make sure you are using a state specific lease.

We also place bait stations in the cellar and around the property in the autumn and make traps (w/ instructions) available to tenants at the first sign of rodents.In our multi-unit buildings on the edge of more  mouse populous neighbourhoods, we have a pest control service monitor and maintain a "trap line" in the properties.In all cases, a coordinated effort between landlord, tenant and {if present} pest control service, is essential to prevent a mouse or two from becoming a colony.  

They seem happy there but based on what they told me about their employment, I started wondering if they are able to pay the rent. 

Anyway Told the tenant to get some traps bait/ poision.

West Trenton 1.5 month security deposit and application review of rental history, employment/income verification,eviction and credit checks, etc.

The credit report may also include additional elements such as possible place of employment or current and past places of residency.

In a recent survey by TNS Custom Research, 51% of Americans said they would like to be self-employed, however many of them didn’t take the steps necessary to achieve those dreams. 38% cited the fear of going bankrupt as the number one reason they never took action, 37% cited the fear of an irregular income, 15% said they didn’t have enough capital & 8% said they lacked the necessary skills.

That's as someone who is happily employed, much like yourself. 

However, his employer is going to reimburse him for his PM fees because of the job move, but in order for that to happen, they need invoices from a company stating the fees so this method wouldn't really work for that.