GetMembers.cs

void Main()
{
    Mapper.Initialize(cfg =>
    {
        cfg.CreateMap<Source, Destination>().ForMember(dest => dest.ValuesCount, opt => opt.MapFrom(s=>s.Value1+s.Value2));
        cfg.CreateMap<SourceItem, DestItem>();
    });
	Mapper.AssertConfigurationIsValid();
	//Mapper.Configuration.BuildExecutionPlan(typeof(Source), typeof(Destination)).ToReadableString().Dump();
	var expression = Mapper.Configuration.ResolveTypeMap(typeof(Source), typeof(Destination)).PropertyMaps.Last().CustomMapExpression;
	MemberVisitor.GetMembers(expression).Dump();
	var source = new Source { Values = new List<SourceItem> { new SourceItem { Value = 5 }, new SourceItem { Value = 10 } }, Value1 = 3, Value2 = 3 };
    //Mapper.Map<Destination>(source).Dump();
	Mapper.Map<DestItem[]>(source.Values).Dump();
}

public class SourceItem
{
   public int Value { get; set; }
}

public class DestItem
{
   public int Value { get; set; }
}

public class Source
{
   public IList<SourceItem> Values { get; set; }
   public int Value1 { get; set; }
   public int Value2 { get; set; }
}

public class Destination
{
   public IList<DestItem> Values { get; set; }
   public int ValuesCount { get; set; }
}

class MemberVisitor : ExpressionVisitor
{
	private List<MemberInfo> _members = new List<MemberInfo>();

	public static IReadOnlyCollection<MemberInfo> GetMembers(Expression expression)
	{
		var visitor = new MemberVisitor();
		visitor.Visit(expression);
		return visitor._members;
	}
	
	protected override Expression VisitMember(MemberExpression memberExpression)
	{
		_members.Add(memberExpression.Member);
		return base.VisitMember(memberExpression);
	}
}