The command iwlist scan
will tell you, but it won't decode it for you. You have to look at the frequency, channel, and offered speeds to guess what variety of wifi is offered by that access point.
Cell 11 - Address: 00:24:A8:1B:CC:00
Channel:7
Frequency:2.442 GHz (Channel 7)
Quality=28/70 Signal level=-82 dBm
Encryption key:off
ESSID:""
Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s
9 Mb/s; 12 Mb/s; 18 Mb/s
Bit Rates:24 Mb/s; 36 Mb/s; 48 Mb/s; 54 Mb/s
Mode:Master
Extra:tsf=000000cd962216d3
Extra: Last beacon: 2328ms ago
That's a 802.11g access point. You can tell because the frequency is 2.4GHz, and the speeds go up to 54Mbs.
802.11n will be on either 2.4GHz or a 5GHz channel, but the speeds offered will be higher.
802.11a will be on a 5GHz channel, but with speeds less than offered by n.
Also pay attention to the security information.
IE: WPA Version 1
Group Cipher : TKIP
Pairwise Ciphers (1) : TKIP
Authentication Suites (1) : PSK
versus:
IE: IEEE 802.11i/WPA2 Version 1
Group Cipher : CCMP
Pairwise Ciphers (1) : CCMP
Authentication Suites (1) : PSK
IE: WPA Version 1
Group Cipher : CCMP
Pairwise Ciphers (1) : CCMP
Authentication Suites (1) : PSK
And others. Some wifi stacks don't talk well to some WPA standards.