Defined terms: Ensure references to parties, documents, issues etc. are consistent. If defined terms are used (e.g. Company X, the Agreement, the Expert), ensure they are used consistently and in accordance with house style. Note that firms have different ways of formatting defined terms, for instance: (Company A), (“Company A”), (Company A) etc. Also check that the document includes a comprehensive list of all defined terms, correctly set out in alphabetical order.
Party names: some clients are very specific about how they want their company’s name to be written. Some may insist on “Plc” or “Ltd” at the end, others may care less. Check that a party’s name is used consistently and in accordance with the client’s preferences.
Dates: check how the firm expects dates to be written. Some would use 1 January 2019, 1 Jan 2016 or 1st January 2019, whilst others would use 01/01/2019 or 01.01.2019. Keep dates consistent and in line with house style.
Numerical figures: ensure financial figures are referred to consistently (e.g. £1,000,000; £1 million; or £1m. Not a combination of the three). Also (where appropriate) check that numbers are accurate, for instance by referencing the financial accounts from which those numbers were extracted.
Punctuation: check the firm’s policies regarding punctuation. For instance, you may need to ensure that the end of each sentence that forms part of a list has a semi-colon (with the last sentence in the list being followed by a full stop). Before sending off a long document to your supervisor, control+f “; or” and “; and” to quickly check whether your lists are correct after amending a document. It’s easy to cut and paste paragraphs and then forget to amend the position of what was intended to be the punctuation for the penultimate line of a list.
Format/structure: ensure the document adheres to house style. For instance, check that indented sentences, bullet points, headings etc. are all formatted and justified consistently and in adherence with house style.
References/citations: firms may have a particular way of referencing cases/statutes. Some may want the party names to be underlined or italicised. Some may want cases to be referenced only in footnotes, whilst others may prefer you to include references in the main body of text. Consistency is key.
Check references: documents tend to evolve significantly over time. This means that sections may move around a lot and paragraph/section numbers may change. For this reason, it is imperative to carefully check that cross references, footnotes and citations throughout the document are accurate and that no content has been accidentally left in from old versions/templates. There may also be phrases in a document that have not been referenced but may require referencing (especially documents used in litigation proceedings, such as witness statements). Where this is the case, it may be worth flagging this to your supervisor.
Square brackets/blobs: sometimes when a document is in its early stages, the creator will use symbols/indicators such as  or [ref needed] to denote the need to subsequently include information that is not yet known or available. If proof reading a “final” copy of a document, it may be worth using control+f to look for these kinds of indicators in case there is information yet to be included.