Q: How do you know if you're saturating colours too far, so that they'll be unprintable?
Well, that's not always easy.
Pushing colours beyond the gamut of the printer is clipping, essentially. Like images, printers have a finite range of colour in each channel (either 0-255 for RGB printers, or 0-100 for CMYK printers).
If your image's colour space is equivalent to (or smaller) than the gamut of your printer, then it's easy - if a channel reaches 0 or 255, you've gone too far. For example, if you work in sRGB, and your lab's print gamut is sRGB-esque, then the numbers should be roughly equivalent, and therefore fairly reliable. (Not 100% reliable, though - very bright colours eg clothing can exceed print gamut.)
But if you're editing images in Adobe RGB, then the brightest possible colours in your image's gamut might exceed the brightest possible colours in the print gamut, at least in some areas of the spectrum. And if you're editing in ProPhoto RGB, then there's no doubt at all that the brightest possible colours in your image's gamut will exceed those of your lab's gamut.
So, in that case, you need to soft-proof. Your lab should be able to provide you a soft-proofing profile on request (many will have them available on their websites). Set up the soft-proof according to the lab's instructions, and toggle it to see if any colours are going to suffer when printed.
If you see that you've saturated too far, you need to return to the saturation adjustment layer, and reduce it until the colours are acceptable. Exactly how you do this will depend on the nature of the adjustment - you'll need to either reduce layer opacity, or adjustment strength. (more)
Once you're familiar with your screen and your printer, you'll often instinctively know how far you can push certain colours.
Note: Don't simply check soft-proofing without inputting the exact profile and settings recommended by the lab. Photoshop's default soft-proof settings are CMYK, which (usually) have no relevance to photographers, and will make the problem seem worse than it really is.