Detect if app is running on a macOS beta version

I believe you're out of luck. About This Mac uses PrivateFrameworks/Seeding.framework, here the important disassembly:

/* @class SDBuildInfo */
+(char)currentBuildIsSeed {
    return 0x0;
}

So it seems this is a build time compiler flag. The plists in the framework don't contain this flag unfortunately.

Sample private API usage: kaloprominat/currentBuildIsSeed.py

For the crazy ones: It would be possible to read the binary and compare the assembly for the function. I'd start with the class-dump code, which gets you different fat binaries and the function offset.

This is far from perfect but macOS BigSur release notes mention:

Known Issues
In Swift, the authorizationStatus() property of CLLocationManager is incorrectly exposed as a method instead of a property. (62853845)

This^ is new API introduced in MacOS11 / iOS14

So one could detect this for this particular beta.

import CoreLocation
func isMacOS11Beta() -> Bool {
    var propertiesCount = UInt32()
    let classToInspect = CLLocationManager.self
    var isMacOS11OrHigher = false
    var isMacOS11Beta = false
    let propertiesInAClass = class_copyPropertyList(CLLocationManager.self, &propertiesCount)
    if classToInspect.responds(to: NSSelectorFromString("authorizationStatus")) {
        isMacOS11OrHigher = true
        isMacOS11Beta = true
        for i in 0 ..< Int(propertiesCount) {
            if let property = propertiesInAClass?[i], let propertyName = NSString(utf8String: property_getName(property)) as String? {
                if propertyName == "authorizationStatus" {
                    isMacOS11Beta = false
                }
            }
        }
        free(propertiesInAClass)
    }
    return isMacOS11OrHigher && isMacOS11Beta
}