You can palpate a blood pressure.
You HAVE to use the cuff.
1. Find a strong radial pulse.
2. Inflate cuff 10-20 past the point where the radial disappears
3. Release the pressure (slowly!) on the cuff while palpating the radial pulse.
4. At the point that the radial resumes that is the presumed systolic.
5. There is no way to get an accurate diastole by this method.
* I feel (the radial) on the inflation as a marker of accuracy, but the deflation # is usually real close. I generally check both arms as time and clinical presentation allows.
Now, other options.
I always use a doppler at the brachial in a manual pressure if possible. The doppler that we carry on the aircraft plugs into the ICS and allows us to hear via our helmet headsets. If you are not in the HEMS environment, it should be fairly straightforward with an available doppler. This would kinda render the Reading/Palp a bit useless.
Now, as to rules of thumb for perfusion and assumption of SBP.
IF I have a Radial Pulse, I generally have a SBP of at least 80.
IF I have a Femoral Pulse, I generally have a SBP of at least 70.
IF I have a Carotid Pulse, I generally have a SBP of at least 60.
None of these "assumptions" are exceptional, but a "quick and dirty" measure. I still try and get an accurate baseline. Now, in actual practice IF I am missing a radial I generally need to proceed to immediate intervention to maintain perfusion. Also, ALWAYS, compare and check L & R at the same time. Also, remember that elevated HR in the presence of a low BP is not a reassuring finding - generally, (not always) you need to move a bit faster. The main thing to remember is to treat the PATIENT not any # - good assessment is the key to good patient management!
Hope this helps!