How to know if an aircraft is climbing or descending?
Listen. Climbing aircraft need more thrust, so the noise they produce is related to their vertical speed. Since most excess performance is available at low speed, a slowly flying aircraft producing a lot of noise will most likely be climbing. Once you gain a little experience, the pitch of the engine and propeller sound of piston-engined aircraft will be a very good indicator of their thrust setting.

This doesn't work for gliders. Here you need to wait to be sure if they are climbing or descending. Only when you see that the glider flies with speed brakes extended you can be sure instantly that it is descending.