if (User.HasClaim(ClaimTypes.Role, "Admin")) { ... }

[ClaimsAuthorization(ClaimType="a", ClaimValue="b")]
public IHttpActionResult Get()
{
    return Ok();
}

public class ClaimsAuthorizationAttribute : AuthorizationFilterAttribute
{
    public string ClaimType { get; set; }
    public string ClaimValue { get; set; }
 
    public override Task OnAuthorizationAsync(HttpActionContext actionContext, System.Threading.CancellationToken cancellationToken)
    {
        var principal = actionContext.RequestContext.Principal as ClaimsPrincipal;
 
        if (principal.Identity.IsAuthenticated &&
            principal.HasClaim(x => x.Type == ClaimType && x.Value == ClaimValue))
        { 
            // User is authorized, complete execution
            return Task.FromResult<object>(null);
        }
 
        actionContext.Response = actionContext.Request.CreateResponse(HttpStatusCode.Unauthorized);

        return Task.FromResult<object>(null); 
    }
}