Solid Resources needs a way to tell what their containers are

The Web Access Control Spec has a section on how to calculate the Effective ACL Resource by following a resource up to its "container resource".
The Solid Protocol Spec specifies using 3.1 URI Slash Semantics. So it is clear what the intention is.

But how is a client to know it has landed on a Solid server that implements those intentions rather than one that has not? There will be many LDP servers implemented by the LDP consortium members. A client can't distinguish between those servers and those implementing Solid, potentially leading to many unnecessary requests.

There are a number of ways of doing this.

1. Compatible with LDP servers: every resource has a reverse link to the container, which in Solid would always be ../, but on other LDP containers could be something else.

Link: <../>; rev="http://www.w3.org/ns/ldp#contains"

2. Specify that a resource is not an ldp:BasicContainer or ldp:Resource but a subset of those, namely a solid:Container or solid:Resource. Note this was proposed in the 2013 to the LDP group and later became issue-50.

Clearly, if a resource specifies that it is a solid resource, then point 1 is unnecessary as the URL structure contains the information about the ldp:contains link. On the other hand, for more lax servers following LDP, then 1 is needed.

I don't think that having an "acl" link header is enough to distinguish the server as being a solid one, as that link could very well also be used by LDP servers.

Read the Web Access Control Spec section on calculating the Effective ACL Resource and the Solid Protocol Spec section 3.1 on URI slash semantics, then compare the proposed reverse-link and resource-type approaches. Done means reaching a decision on how clients distinguish Solid containers and recording that decision in the relevant specification.