Ps. I am new in altcoins. Thanks in advance!
the first thing you need to look is the volume for 24 hours from different exchange
you can check it in coinmarketcap.com and the next is the price graph in the last few days
for me i pick the coin that have a drop and rise graph and monitor it to get atleast the best price to sell and buy
but doing it will not gain a lot of profit and its also stressful
so in my opinion its better to invest in the coin you think have lot of potential and hold long term