Proofs are standardized products that are created and tested according to a certain set of values. This is exactly the point that distinguishes them from any “colourful printouts”.
Specifically: A proof for coated printing paper is produced according to the standard values of ISOCoated V2 (paper type 1 and 2, glossy and matt coated image printing, dot gain curves A (CMY) and B (K) from ISO 12647-2:2004) and checked according to a set of values (FOGRA39L). A proof for uncoated paper (e.g. PSOUncoated or ISOUncoated) is produced and checked according to completely different value sets. Logically, because a print on uncoated paper looks definitely different in terms of colour and white value than a print on picture printing paper.
A proof must therefore always be prepared according to a standard and be verifiable according to a reference value set. A list of the current Proof Profiles (as of 2012) can be found here.
The problem: Many printing processes such as digital printing on a color laser or printing on a large format printing system (LFP) are not standardized and therefore there are no valid profiles and specifications.
So what to do? The most frequently used standard has established itself as the “de facto basis”: ISOCoated V2.
This is understandable, because colour-critical prints, catalogues etc. are mainly produced in offset printing on picture printing paper and are therefore subject to this standard. It is therefore generally assumed that a digital printer or an LFP printer, for example, should follow this standard and at least achieve this colour result.
So if you need to make a proof but don’t have the exact details of the profile you need, proof ISOCoated V2, which has become the industry’s most widely used standard and will always be accepted as the basic proof.
Unfortunately, a proof without a profile cannot be produced, because that would just be “colored paper from a proofing system”, but not a valid, ISO-compliant proof.