This question already has an answer here:
How I will come to know that synchronization is complete. Also what is potential rewrite attack?
I am using
geth --testnet --fast --cache=1024
last couple of results
I1026 08:43:50.011852 eth/downloader/downloader.go:711] Peer b274dd4c49d951d2 [hs 0.00/s, bs 0.00/s, rs 0.00/s, ss 0.00/s, miss
0, rtt 20s]: potential rewrite attack: #1715594 [00000000…] <= 1715594 limit I1026 08:45:18.157176 core/blockchain.go:1001] imported 1 block(s) (0 queued 0 ignored) including 18 txs in 19.219834ms. 1805595 [11e04fc8 / 11e04fc8]