How can I validate an enum using FluentValidator?

You've come very close to the solution, you can use Must to check the type of your fields or move them with others fields or in your case with a enum ,

To know if the type of a field is equal to a particular type you first need to capture the type of the field with GetType() .

RuleFor(x => x).Must(p => p.Type.GetType().IsEnum);

IsEnum , already does the check you need, Fluent Validation has an error condition to be triggered when the result is TRUE , so if you want an error to be thrown when your property is different from IsEnum then use a denial.

RuleFor(x => x).Must(p => !p.Type.GetType().IsEnum);

For a check in a client class it would look like this.

using FluentValidation;
using System;

namespace TesteDryIoC.Generic.Validacoes
{
    public class ClienteValidator : AbstractValidator<Cliente>
    {
        public ClienteValidator()
        {
            RuleFor(x => x).Must(p => p.Type == MeusEnuns.Numero);
            RuleFor(x => x).Must(p => p.Type.GetType().IsEnum);
        }
    }
}

In RoleFor , I'm accessing my client class and passing all its properties to Must, you can do several checks like this or even call a more robust method returning a bool with the result of the validation.