Validate latitude and longitude

I want to validate the latitude and longitude. Right now, I check just so that the value is not empty, but I want a validation to check that it is a valid latidue or longitude.

How do I do that?

My property looks like this:

public string Lat
{
    get {
        return this._lat; 
    }
    set 
    {
        base.ValidationErrors.Remove("Lat");

        if (String.IsNullOrWhiteSpace(value))
        {
            this.ValidationErrors.Add("Lat", strings.Positions_Lat_Empty);
        }

        this._lat = value != null ? value.Trim() : null;
    }
}

public string Lng
{
    get { 
        return this._lng; 
    }
    set {

        base.ValidationErrors.Remove("Lng");

        if (String.IsNullOrWhiteSpace(value))
        {
            this.ValidationErrors.Add("Lng", strings.Positions_Lng_Empty);
        }

        this._lng = value != null ? value.Trim() : null; 
    }
}