How do you tighten bolts on Kohler toilet seats the bolt has a plastic cap?

Here's two things:
Look at the lid, with the seat down. The bolts are sometimes covered with a snap-in plastic cap or flap. If so, pry up with a screwdriver. Then use a screwdriver and wrench to tighten.
If the bolts appear to be cast as a part of the hinge assembly, you should be able to reach around the seat to the back side (no pun intended) of the toilet, and tighten with a wrench.
I sometimes use a 'mirror trick' to get a better idea of what's back there. Just use a small mirror and hold it to where you can see the bolt and not on the back.

If the bolts are plastic or metal you will need to cut them off and install new ones from new seat or bolt kit there is no easy trick its just an easy cut them off or you could try some wd-40 or nutbuster.

If you are trying to install a standard toilet seat on a special "smooth sides" toilet, it likely will not work. Most hidden bolt toilets come with a special seat because you cannot get to anything from below. This special seat has a set of expanding style bolts that are attached to the screw that is part of the seat. The plastic expanding bolt goes into the hole(s) on the toilet, and as you tighten the screw on the toilet seat bolts, they expand the plastic part in the hole of the ceramic toilet.

No do not use metal bolt and nut as this can crack the toilet. Just go to Lowes or Home Depot and buy 2 plastic bolts , 2 plastic wing nuts, and 2 plastic washers. Install with seat through toilet holes and tighten by hand until tight. I then use pliers to tight just a turn tighter and the seat should be center as you tightened one side, then the other, and back again until tight.

Usually there are plastic caps that snap down over the bolt head.
Shut toilet lid and get down close with a flashlight.

Seat attaches in two places.
Reach underneath and you can feel two bolts with nuts.
You don't need to see them, just feel with your hand.

The bolt head is covered with a plastic cover.
The cover is added for aesthetic purpose only.
Use small screwdriver or fingernail file or nail or scissors to pry up the cover and reveal bolt head.

Use bigger screwdriver to tighten bolts.
You may have to keep one hand on underside to hold nut while turning bolt clockwise.
Another trick is use hand to push bolt all the way up, and then finger tighten bolt most of the way, and use screwdriver for last few turns.

Tighten bolts good and tight, but don't overdo it.
Bolts will come loose again and you can tighten more later.

I repaied one of these laast week and it had the Plastic nuts in between the webbing on the seat where the lid connects I believe You remove the plastic cap with a knife and use a flat tip screw driver to remove the screws and replace the seat.

Most generic seats have a thin plastic cap that snaps in place over the bolt heads just behind the seat. Normally they hinge backwards so carefully pop them open with a flat blade screwdriver. Sometimes you can tighten the bolts from underneath even.

Roberta3fan could be giving bad advice based on experience with other toilets. This does not seem to be an option for this toilet design. I am in a similar situation and it appears that the nut (and it is a metal bolt) is not what you would expect. Here is a picture of the anchor kit for this toilet seat: http://www.plumbingsupply.com/images/kohler-anchorkit-84999.jpg I am going to assume that the sleave that should be engaging the bolt is cross threaded and try to grab it with a pair of vice grips. I'll let you know if it works.

I have the same low boy where the bolts are inside the hollow body of the toilet. I called Kohler customer service and was told to use brute force to yank the toilet seat out. My husband suggested using a saw to cut the bolt but he hasn't done it yet. He wants to replace all our Kohler low boys which constantly leak becos of poor design of handle and flapper. The handle pulls the flapper slightly to the side when you flush. And the flapper lip being a tiny bit too small, if it doesn't sit just right, the toilet leaks. We've replaced one already with a Toto and love it. Too bad that Kohler isn't trying to fix the problem as I see many many online posts with the same problem and recommendation to replace the Kohler with Toto or somethng else.