oven cleaning help and tips
Look in the oven. Smell the oven. Turn the oven (and exhaust fan, if any) on to the maximum temperature. If you smell nothing untoward (there might be some "normal" smallish amount of smoke/burnt smell when you fire it up hot for a while) it's fine.

If it smells like rodent urine, or you see rodent scat, you might have a problem, and firing it up might smell very bad and/or cause rodents to come running out of the insulation. Other than that, most issues of disuse will be cured by heating it throughly, once.

Since few, if any baked items come into direct contact with the oven (most are in a pan on a rack) there's no health/safety difference between a shiny clean oven and a fairly filthy-looking oven. Most oven cleaning chemicals are far scarier than what's baked on the oven; and all self-cleaning ovens need is to be run hot enough to burn off whatever is on them.