In CMS 12 configuration settings are exposed as options classes which are already registered in the IOC container. 

I haven't tried this but you should be able to simply inject IOptions<SchedulerOptions> as such

public TestClass(IOptions<SchedulerOptions> options)
    {
        _options = options.Value;
    }

You can then use the value for _options.Enabled for your logic

See below documentation for details 

https://docs.developers.optimizely.com/content-management-system/docs/configuration

https://docs.developers.optimizely.com/content-management-system/docs/configuring-cms#scheduleroptions

I can confirm this approach works.

In addition, there is a transformation step in the DXP deploy that force sets this value to Enabled = true for the scheduler instance and Enabled = false for all other instances (valid for preprod and prod), which means that you can trust this setting. If it is enabled, you're on the scheduler. Do note though that there is no scheduler instance in integration, but unless you deliberately change the value, it will be correct for integration too, since scheduler is by default enabled.

To visualize what both Ronil and Johan said, check the scheduler property

Or if you prefer the programmatic approach

services.Configure<SchedulerOptions>(options =>
{
    options.Enabled = true;
});

And use something like this in your code

public class SchedulerEnabled
{
    private readonly IOptions<SchedulerOptions> _options;

    public SchedulerEnabled(IOptions<SchedulerOptions> options)
    {
        _options = options;
    }

    public bool Value { get { return _options.Value.Enabled; } }
}