You can determine whether a file on your local system has changed compared to the version hosted on Wasabi and, therefore, needs to be uploaded again. You can generate an ETag locally and compare it with the ETag that is generated on the uploaded objects on Wasabi.
For objects that are not multipart: The ETag is simply the textual representation of the MD5 checksum of the file.
For multipart uploads: Calculate the MD5 checksums corresponding to each part. For example, if you uploaded a 14 MB file and your part size is 5 MB, calculate the three MD5 checksums (the checksum of the first 5 MB, the second 5 MB, and the last 4 MB). Then, take the checksum of their concatenation. Since MD5 checksums are hex representations of binary data, make sure you take the MD5 of the decoded binary concatenation, not of the ASCII or UTF-8 encoded concatenation. When that is done, add a hyphen and the number of parts to get the ETag.
Refer to What is the algorithm to compute the Amazon-S3 Etag for a file larger than 5GB?
A multi-part object can have one part. In that case, the ETag is NOT the MD5 checksum of the file. Instead, it is the MD5 checksum of the textual representation of the MD5 checksum of the file, followed by "-1".