How to know if a DateTime is between a DateRange in C#

Nope, doing a simple comparison looks good to me:

return dateToCheck >= startDate && dateToCheck < endDate;

Things to think about though:

• DateTime is a somewhat odd type in terms of time zones. It could be UTC, it could be "local", it could be ambiguous. Make sure you're comparing apples with apples, as it were.
• Consider whether your start and end points should be inclusive or exclusive. I've made the code above treat it as an inclusive lower bound and an exclusive upper bound.

Usually I create Fowler's Range implementation for such things.

public interface IRange<T>
{
    T Start { get; }
    T End { get; }
    bool Includes(T value);
    bool Includes(IRange<T> range);
}

public class DateRange : IRange<DateTime>         
{
    public DateRange(DateTime start, DateTime end)
    {
        Start = start;
        End = end;
    }

    public DateTime Start { get; private set; }
    public DateTime End { get; private set; }

    public bool Includes(DateTime value)
    {
        return (Start <= value) && (value <= End);
    }

    public bool Includes(IRange<DateTime> range)
    {
        return (Start <= range.Start) && (range.End <= End);
    }
}

Usage is pretty simple:

DateRange range = new DateRange(startDate, endDate);
range.Includes(date)

You could use extension methods to make it a little more readable:

public static class DateTimeExtensions
{
    public static bool InRange(this DateTime dateToCheck, DateTime startDate, DateTime endDate)
    {
        return dateToCheck >= startDate && dateToCheck < endDate;
    }
}

Now you can write:

dateToCheck.InRange(startDate, endDate)