Subject pretty much says it all.
I have a few photos that I believe are good enough to print fairly large. But color spaces for printing can be a tangled web to sort. What do people here do? For something large and expensive, I was thinking of taking a crop of the photo and getting that printed the exact way I would, except as a small size, like 4x6, or something affordable. Then maybe tweak and reprint?
I don't have a fancy monitor anymore, just on a Mac laptop right now with a cheap Canon ink jet printer.
Short answer
IMO instead of getting a 4x6" print of a small crop, maybe get like an 11x14" print of the full image, from the same service, on the same paper, etc. Unless and until you check and report what the service you intend to use accepts, it's premature to discuss sRGB versus Adobe RGB versus other color spaces; and/or JPEG versus 8-bit TIFF versus 16-bit TIFF.
Full answer
Alas, what you in particular can realistically do will not get anywhere close to the
best way to preview before using a printing service.
The best way to preview before using a printing service is to use one sophisticated enough to let you:
(1) download an ICC printing profile for the specific one of their printer + inks + driver settings + paper combinations that they will use to print your photo;
(2) use it to soft-proof your photo on your calibrated and profiled monitor;
(3) with that, pick the rendering intent of your choice, and black point compensation or not, based on what you think looks best, and maybe even tweak your photo to get the most out of what the print can actually show;
(4) perform a convert-to-profile with those chosen settings; and
(5) send the service your file encoded in their ICC printing profile, and tell them 'no corrections'.
Historically laptops have had quite mediocre screens, although that has changed somewhat for the better in recent years, and maybe your Mac laptop has a serviceably-good one. Historically most monitors came far from 'correctly' set, although that has changed somewhat for the better in recent years, and maybe your Mac laptop will be pretty close even without being calibrated and profiled.
IMO the problem with a small crop is that it isn't good for judging much besides detail, sharpness, and noise in that part of the image. It's not often good for judging overall color balance, contrast, etc.