In an HTTP request, whether it's HTTP or HTTPS can be determined primarily from the structure of the request URL itself. 

Here's how:

1. **URL Format**: In a typical HTTP request, the URL is included in the request line. If the URL starts with "http://", it's an HTTP request. If it starts with "https://", it's an HTTPS request. For example:
   - `GET http://example.com/path/to/resource HTTP/1.1` indicates an HTTP request.
   - `GET https://example.com/path/to/resource HTTP/1.1` indicates an HTTPS request.

2. **Port**: While not explicitly mentioned in the request line, HTTPS usually operates on port 443, while HTTP typically operates on port 80. The client connecting to the server on one of these ports gives an additional clue about the scheme.

3. **Transport Layer**: HTTPS requests are encrypted using Transport Layer Security (TLS), while HTTP requests are not. This encryption is negotiated during the initial connection handshake. However, this determination happens after the HTTP request line has been transmitted.

These are the primary means by which the scheme (HTTP or HTTPS) can be determined from the HTTP request lines.