A winning grant is timely, topical and in line with the priorities of the people granting the money. If you are critiquing a grant proposal, make sure that the proposal lives up to these priorities and that the proposal writer has done the necessary research. Your job is to make sure the proposal has the highest possible chance for success.
Read through the grant as a whole first, and make notes on clarity of writing, readability and succinct phrasing. If you have a hard time getting through it, so will the reviewers; this is vital feedback.
Review the guidelines of the grant to make sure that the proposal is in the right format and that all of the materials are included.
Critique the style. The grant proposal should feel urgent, without being overblown.
Check the research. Look at the mission statement of the funding source. Is the proposal going to an organization that actually funds this kind of project? Also ensure that the amounts are within those awarded by the foundation or funding source.
Assess if the proposal is doable. Ensure that key persons have the experience to complete the project and that the funding covers the needs of the project. If the project doesn't seem feasible, rejection is likely.