It is, in theory, possible to be vaguely accurate torquing bolts without an actual torque wrench.
Torque is measured in applied force multiplied times the length of the lever arm. You achieve the same torque applying a 10 pound force to the end of a 6-inch lever as you do applying a 5 pound force to the end of a 12-inch lever. In both cases the torque is pounds times inches -- 60 inch-pounds. (And you can easily do the conversion to foot-pounds or newton-meters or whatever.)
Measuring the length of the lever is fairly easy, of course (though you do need to take note of where on the lever you apply the force -- the "length" is from that point to the pivot point). Getting a good estimate of force, however, is trickier.
Skilled bike mechanics tend to develop a "calibrated arm", and can feel, within an acceptable margin, how much force they are applying. (Understand that being within a factor of 2 is generally adequate.) The Saturday afternoon mechanic, on the other hand, may not be so skilled.
One could presumably dig out a spring balance and use that to apply the force to the lever (socket wrench handle), but if you're going to that much trouble you might as well get a torque wrench. But, if you have some objects for which you know the weight, it is possible to lift the "reference" weight, getting a "feel" for it, and then apply the same force to the lever. One can even be "sorta" accurate scaling by half or by 2x (though beyond that is questionable).
Note that I'm not saying that such "shade tree" techniques are to be preferred over having the proper tools, just that there is the option, if you are caught in a bind.