My college apartment is furnished with pretty basic appliances and the oven has just a knob to set the temperature, an indication light to show that it's on, and a switch to turn the oven light on and off.
I've only used digital ovens that beeped when the set temperature was reached, but this one doesn't have any sort of indicator. How would I know when the set temperature has been reached?
You don't mention if it is an gas or an electric oven, but if it is the latter, there is generally an easy way to tell if the oven is up to temperature without a thermometer.
Give the oven about 10-15 minutes to heat up then rotate the temperature dial back and forward a few degrees (decreasing and increasing the temperature respectively). Listen carefully, and you you should hear a very slight "Click" when the thermostat kicks in. Immediately stop rotating the dial, and read the temperature off the scale. This will be roughly your oven temperature.
This method won't work with all ovens, gas specifically. It may not work with other exotic electric ovens that don't have a basic bimetallic thermostat either.
Simple enough, really. For an electric oven, look at the elements (easier with the light off - also easier if your oven has a glass window in the door - you don't specify - but not essential.)
If they glow orange and then get dark, they are cycling on and off. If the "off time" is more than a minute or so, it's likely where it thinks it's set. My current one does not just slam the elements full on until it hits temperature, so I'd not say turning off at all is a good indicator.
That may have a wide relationship to what the actual temperature is, so a thermometer is still a good idea, but you can live without one if you look at the food and it seems to be too brown/black or not browned enough, allowing you to infer the temperature is higher or lower than it says, and change the setting next time.
In many cases you can also hear the elements click on and off. Useful if there's no window.
If a gas oven, look at the burner flames. You can usually hear those, as well. Most will only go back to "large pilot" once they hit temperature, so the main flame dropping is a good sign you are at temperature.
Ye Olde Recipes (1980's and before) generally just said something like "preheat for 15-20 minutes," so that's another simple method, as is not even preheating (so, you bake a little longer, but the oven is on for less total time) for the many things that don't really need it, reserving preheat for the few that do.
The answer to this is simple, and is true for any oven: use an oven thermometer.
That's one, there's lots of other options - not endorsing any specific one (the link goes over several).
Oven temperatures aren't terribly consistent even in relatively new ovens, so it's always ideal to use an oven thermometer to see what the actual temperature is, assuming you have something that cares about the actual temperature cooking (like a cake or cookies).
The best ones can sit outside the oven and just use a probe inside it, so they're very readable, and can have alarms set to alert you when it's ready.
A way to avoid the trouble is just to put in the dish while the oven is heating up. If you develop a feeling for cooking, this will be fine with almost any type of dish. The benefit of preheating is mostly so that you can follow a recipe to the letter, including total oven time. But you can go by eye/smell too. There are a few dishes where inside temperature/cooking time may ideally be slightly different from the outside temperature but I wouldn’t worry about that too much. The benefit is that it also saves energy and costs because no heat is wasted.
