Leak check mini split AC
This is really for licensed people, the system needs to be evacuated prior to charging.
If you have the correct adapters you could pressurize the system to 120 psi with nitrogen (don’t use air it is dirty and wet) let the system sit for 15 minutes to see if you loose pressure this is a gross leak test and can identify larger leaks.
The hvac guys will need to pull a vacuum on the system to remove the air (non compressible in hvac terms)  and any moisture , they will usually do a leak back check no change in pressure then charge the system and checks for leaks with a detector.
We use nitrogen for leak checking because it is clean and dry. Using air from your home compressor can contaminate the line set with moisture and possibly compressor oil. Other than this if you have a vacuum pump you could pull a vacuum on the system , EPA recommends a base pressure below 500 microns of vacuum I like keeping a new system at high vac for several hours or even overnight to fully remove any moisture. Anything beyond this would require a licensed technician in most countries. In the US fines up to $25,000.00 For unlicensed person charging a system, the compressor has the charge opening that valve and allowing Freon to fill the line set is charging the system.
So to stay safe don’t go any further than a pressure / vacuum leak test on your line set.