First, on whichever line you want to use, you need to identify *ALL* the sockets and fixtures that are on it. Then, add up all the watts that might be pumped through it on a good day. (NOT necessarily hypothisizing that you'd plug, say, 5 - 1500 watt hairdryers in and turn them all on at once.! Just figure what you MIGHT have running on that 1 line.)
Then, convert all your 'units' to the same standard, say watts. The calculation is: Watts = Volts x Amps. (Power = Volts x Current). So, in a 120v system, 2000 watts =~ 17 amps. 1500 watts =~ 12.5 amps. (like your hairdryer.) Then, add in the watts for your fan. If the 'average' load on the existing circuit + your fan > Total Amps on the circuit, you'll need to find an alternate source.
My best to ya and hope this helps.
PS: I'm not an electrician but this is how I do it at home.