#1 is yourself - if you really feel like cr*p, you probably need a day off the bike (or at least take it easy that ride). If you feel GREAT!, go out and hammer. #2 - with the cyclecomputer you have now, you can track long term changes in performance over the identical route - I've used a 4-5 mile section of a local bike trail and a couple of local hills for years, and started tracking my commute when it went from 4 miles to 11 each way. When the 5 minute climb becomes a 4 minutes climb, or you cover the section that took 20 minutes & a tailwind now in 15, you KNOW you're getting better. Centuries (and other distance rides) are another good year-to-year measure. Group rides and races are the ultimate arbiters if you want to compare yourself to others vs your personal bests. In fact, riding with another cyclist a bit stronger than myself is the best motivation to push myself - it's mentally hard to push yourself solo, especially longer efforts.
HRMs and power meters are great for pacing yourself during an activity, along with RPE. HRMs are cheaper and good enough for many people. I used one for several years. Power is the current ultimate, but you pay for it ("cheapest" reliable way is a wired or wireless powertap wheel and a unit that can receive and record ANT+ data; there is an iPhone dongle and several bike computers, Garmin watches/computers, and other devices that do it). $500-$600 or more for the wheel, maybe less if you find a racer who was meticulous about his/her gear who's getting new wheels or switching to a different power system (what I did). Adding a cadence sensor may also be useful.