Are you dry hopping? If yes skip the next paragraph, if no read it.
If you're not dry hopping, just skip secondary altogether. Use your spare carboy to ferment MORE BEER
Autolysis takes a while to occur, so you're good up to a month even in the primary before it starts to affect flavor. By then it's going to be fermented out and ready to keg/bottle.
If you're dry hopping then you'll wanna secondary. You'll also want to secondary if you're trying to achieve a super clear beer and want to let it rest for a while. You can only let it sit in primary so long (about a month) before the chance vegitative flavors can develop.
Anyway in general beer is ready for secondary after fermentation is complete. There is no visual way to determine this, so don't try :P. Instead use a hyrdometer to take readings every other day. Once you get the same reading twice, fermentation has completed (or stalled). Assuming you're at your target final gravity (and not experiencing stuck fermentation) you're ready to rack to secondary.
The secondary is not a place for fermentation. People often call it a secondary fermenter, or secondary fermentation, but the truth is that fermentation is complete before it's switched. Old school methods say to transfer it as soon as it slows, but I don't really believe that's necessary. I believe it stems from being afraid of autolysis, which has been proven to take a while to occur, and hence there isn't really any huge hurry to rack to secondary, or a need to secondary at all aside from the two previously mentioned reasons.
Eventually you'll worry less about fermentation and it won't be so hard to let it sit for a few weeks. Especially w/multiple batches going. You'll eventually stop using a hydrometer most likely and just sort of know it's going to be ok.
Hope this helps!