How do I know if I need a new toilet?

It might not be your toilet, you might have a clog in the drain system. Call a plumber and find out. If the toilet is old - it might be the toilet too.

Typically flushing problems have nothing to do with the toilet. If it is just one toilet, there is something partially blocking the drain. Get a good plunger and plunge a few times. If this does not clear the problem, then "snake" the drain. You can buy this tool and do it yourself. Or, if that does not work, call a plumber to see if your main drain is clogged.

increase the water into the toilet

Water pressure to the toilet could be your problem. Not enough thrust to do the job right. Check & see if your valve is open all the way from underneath your toilet & check the valves all the way back to your main line leading outside to the water main. If they're all wide open & you're still not getting the right pressure, you might contact your water company to see if they can come check your line & ask them their opinion in installing a pressure tank in your line. It's the size of a gallon jug & it hooks straight into your water line beside your hot water tank to increase pressure to your house. It will make a big difference in your toilet, sinks & shower. Good Luck

I had the same problem, the flush in the toilet cistern had gone. Got it replaced, flushes even better than before.

My problem was a combination of 2 different issues......roots had grown into the drainage pipe, which need to be roto-routed, and the water level was too low, which was adjusted inside the tank.