From my experience the most network problems are on Layer 1 (Physical layer aka "The cable"). Cisco switches provide a lot of information of the switch interfaces and typically I check:
swt-3750g#show interfaces counters errors swt-3750g#show interfaces status swt-3750g#show interfaces GigabitEthernet x/x/x
Align-Err Malformed frames, bad hardware, duplex mismatch, half duplex collisions FCS-Err Bad frame checksum, duplex mismatch, bad hardware Xmit-Err Speed settings mismatch Rcv-Err Duplex mismatch, backplane congestion UnderSize Received frames which are less than 64 bytes OutDiscards Occur when transmit buffer is full and packets are dropped Single-Col Half duplex, duplex mismatch, high utilization of port Multi-Col Half duplex, duplex mismatch, high utilization of port Late-Col Cable length, duplex mismatch Excess-Col Switch rehad 16 collisions in row, high utilization of port Carri-Sen Duplex settings Runts Frame is smaller than 64 bytes and has bad CRC, duplex settings, physical port Giants Frame is larger than 1518 bytes and has bad FCS, device source
For me, the quick info above is okay before I look into the details and start troubleshooting. Cisco has a good document on their website at Troubleshooting Switch Port and Interface Problems which covers every aspect.