Every fastener on your motorcycle has a recommended torque; you'll find the values for specific ones in the service manual, but you can spitball it based on the fastener's diameter. Use too little torque and the fastener can back out; too much, and it can break off or strip the threads in the hole.
Either way, you'll have to dust off that tap-and-die set. Good torque wrenches aren't cheap, but they pay for themselves quickly.
Torque wrenches range from the inexpensive beam-type to the pricier click-type and digital models probably top the price range. Beam-style torque wrenches are wonderfully rugged but are subject to viewing errors; click-types are as easy as it gets.
Buy one calibrated in both foot-pounds and Newton-meters. Make sure the torque range fits the jobs you need to do. 10 to 85 Newton-meters ought to cover most common tasks. Beware that torque wrenches are most accurate in the middle of their range, so choose a tool accordingly. We can classify the bolts on a motorcycle in two groups, the first group of fasteners is used for bodywork (grade 4.6), the second group is to secure items like the engine and other mechanical items (grade 8.8). In the first group (grade 4.6), and it the thread is dry a M5 bolt maximum torque should be 2.4Nm, at M6 4.1Nm, a M8 9.9Nm, a M10 19.5Nm and a M12 34Nm torque. If the fastener is still from the first group be lubricated you should us for a M5 bolt 1.8Nm, for a M6 3.1Nm, for a M8 7.5Nm, for a M10 14.6Nm and for a M12 25.6Nm of torque.
If the fasteners are from the second group (grade 8.8), to secure items like the engine, you should use different torque values for fasteners with a dry thread you should use for a M5 6.1Nm, a for a M6 10.4Nm, for a M8 25.5Nm, for a M10 50.4Nm, and for a M12 88Nm torque. If the thread is wet/lubricated you should use different torque figures for a M5 4.6Nm, for a M6 7.9Nm, for a M8 19.1Nm, for a M10 37.8Nm, and for a M12 66Nm of torque to secure it.
Of course you always try to verify the torque figures with a experience mechanic. You can see what grade your bolt is by looking at the head of the bolt see picture 2.