s_client.ps1

using namespace System.Net.Sockets
using namespace System.Net.Security
using namespace System.Security.Cryptography.X509Certificates

function ConvertFrom-X509Certificate {
    param(
        [Parameter(ValueFromPipeline)]
        [X509Certificate2]$Certificate
    )

    process {
        @(
            '-----BEGIN CERTIFICATE-----'
            [Convert]::ToBase64String(
                $Certificate.Export([X509ContentType]::Cert),
                [Base64FormattingOptions]::InsertLineBreaks
            )
            '-----END CERTIFICATE-----'
        ) -join [Environment]::NewLine
    }
}

function Get-RemoteCertificate {
    param(
        [Alias('CN')]
        [Parameter(Mandatory = $true, Position = 0)]
        [string]$ComputerName,

        [Parameter(Position = 1)]
        [UInt16]$Port = 443,

        [ValidateSet('Base64', 'X509Certificate')]
        [string]$As = 'X509Certificate'
    )

    $tcpClient = [TcpClient]::new($ComputerName, $Port)
    try {
        $tlsClient = [SslStream]::new($tcpClient.GetStream())
        $tlsClient.AuthenticateAsClient($ComputerName)

        if ($As -eq 'Base64') {
            return $tlsClient.RemoteCertificate |ConvertFrom-X509Certificate
        }

        return $tlsClient.RemoteCertificate -as [X509Certificate2]
    }
    finally {
        if ($tlsClient -is [IDisposable]) {
            $tlsClient.Dispose()
        }
        $tcpClient.Dispose()
    }
}

Just what I needed but I've added a [switch]$Insecure = $false to the parameters and changed to $tlsClient = [SslStream]::new($tcpClient.GetStream(), $false, {$Insecure}) to be able to get certificate information when the certificate is invalid.

thanks you @IISResetMe this is something I was looking for.

Also I found that if the server requires client auth certs, even though you should be able to preview the server certs, this library doesn't provide for that. So I added a optional client auth, too.
[string]$ClientAuth

        #annoyingly (and probably protocol-unnecessarily):  Error retrieving property '$RemoteCertificate': $This operation is only allowed using a successfully authenticated context.
        if ($ClientCert -AND $ClientCert.length -gt 5) {
            $pfxPath = $ClientCert
			$varCached = get-variable -scope script -name 'pfxPassword' -ErrorAction SilentlyContinue
            if (-NOT $varCached) { 
				$script:pfxPassword = Read-Host -Prompt "Enter the password for the client SSL cert PFX file" -AsSecureString 
			}
            $pfxCert = New-Object System.Security.Cryptography.X509Certificates.X509Certificate2($pfxPath, $pfxPassword)
            $sslOptions = [System.Net.Security.SslClientAuthenticationOptions]::new()
            $sslOptions.TargetHost = $ComputerName
            $sslOptions.ClientCertificates = [System.Security.Cryptography.X509Certificates.X509CertificateCollection]::new()
            $sslOptions.ClientCertificates.Add($pfxCert)
            $sslOptions.EnabledSslProtocols = [System.Security.Authentication.SslProtocols]::Tls12
            $tlsClient.AuthenticateAsClient($sslOptions)
        }else {
            $tlsClient.AuthenticateAsClient($ComputerName)
        }