Hello riankins,
and welcome to the forums!
I'll try to answer your questions.
For the first one: Often, desu is very ommitable and only added for politeness. So, often you regulate the politenesslevel by choosing either the plain form (taberu) or the masu form (tabemasu) of a verb. Sometimes you add desu to the plain form to get a plain sentence polite again. especially, I think, in negative sentences. So, you could say "daidokoro ni arimasen" with the masu-form of negaitve-aru so say politely "it's not in the kitchen". You might also say "daidokoro ni nai" (plain-form of negative-aru) to say "It's not in the kitchen" on a more informal level. You might also say "daidokoro ni naindesu", which combines the nai with no desu, and leveling it on a more polite level.
So, often you can omit desu and automatically get more casual.
Anyway: The plain form of desu is da! So, you simply might say da instead of desu, and also get more casual that way.
As for your second question: Very often it's only knowable from context, if you're talking about the present or the future.
Anyway, sometimes it's visible from the grammatical structure. One often cited example is kekkon suru (marrying).
If you want to say, that you ARE married, you have to use the te-imasu structure: "kekkon shiteimasu" would mean, that your ARE married right now.
kekkonshimasu on the other hand would mean, that you are GOING to get married.
So, with habitual, regular or ongoing situations, often the teimasu form shows the present, while anorthr non-past form often indicates a future event.
That's for my thoughts, hopefully someone else will add his thoughts!
Best
kurokuma