You don't need a third-party client to learn grenade lineups anymore. CS2 introduced a native "Practice Mode" with a built-in Picture-in-Picture (PiP) grenade camera, but most players don't know how to turn it on.
How to set up your Sandbox: Instead of queuing for Premier, click Play > Practice > Competitive, and ensure the left-side toggles for Infinite Ammo, Grenade Camera, and Warmup are turned ON.
The Trajectory Ghost: When you pull the pin on a grenade in this mode, a small camera window appears in the bottom left of your screen. This shows you exactly where the grenade will land before you even throw it.
If you are trying to find a new smoke for Mirage Window, simply walk around T-Spawn holding left-click with a smoke. Watch the PiP camera until the landing zone perfectly covers the window, then memorize where your crosshair is aiming.
Once you throw it, you can type
sv_rethrow_last_grenadein the developer console to make the server throw it again, allowing you to run to the site and see if there are any gaps.