I recently installed a generator interlock kit, and am very pleased with the operation.
However, since it is our only panel, one problem arises; there's no real way to tell when utility power is restored, other than manually switching the main back on every now and again.
"Looking at the neighbors" isn't really a good option either because they use a generator as well.
Any recommendations on this?

Do you have the entire service panel fed by your backup generator? Or just some chosen circuits?

(because most backup generators aren't big enough to handle an entire residential service) how many watts is the genset rated as?. and... how many amps is your main circuit breaker rated at?

Since it is an interlock kit, it feeds the whole panel.
It's a Coleman 5000 watt (6250 surge). The generator breaker in the panel is 2 pole 20 amp breaker, ran with 10/3 cable.
Of course, we don't intend to run the entire house off it. We will run one furnace fan (700 watts), fridge, freezer, and a few lights.
A/C, dryer, stove breakers will remain off course.

We just simply won't know when power is restored, that is the problem.

Since it is an interlock kit, it feeds the whole panel.
It's a Coleman 5000 watt (6250 surge). The generator breaker in the panel is 2 pole 20 amp breaker, ran with 10/3 cable.
Of course, we don't intend to run the entire house off of it. We will run one furnace fan (700 watts), fridge, freezer, and a few lights.
A/C, dryer, stove breakers will remain off of course.

We just simply won't know when power is restored, that is the problem.

Indicating when utility power returns is not the only objectionable feature of mechanical interlock kits.
this is a 30A manual transfer switch (5K watts/240V=20.83A) cheapest I googled.
http://www.electricgeneratorsdirect.com/power/manual-transfer-switches-30a-120v.html

here are some 20A manual transfer switches
http://www.electricgeneratorsdirect.com/power/manual-transfer-switches-20a-120v.html

as you can see, these are more expensive than interlock kits, but they are also not disabled and dangerous by simply removing the Panel Cover either.

this inductive pickup is sold as a solution to your situation, it has an audible alarm. With transfer switches, the return of utility power is indicated by EVERYTHING coming back on again.
http://www.heater-store.com/power_back_utility_power_return_9421_prd1.htm

I googled ^^^this thing, I don't know how well it works, I've never used one.

I don't really like interlock kits because circuit isolation can be defeated so easily (basically anyone with a screwdriver) I don't recommend interlock kits and I do not consider them a failsafe solution.