How to check the features which are selected by LASSO I am using LASSO (glmnet) to do feature selection. 
However, how can I check which features are selected?
 A: Use the coef function on the glmnet model.
You will need to choose a lambda value, as different lambdas will give you different feature sets.  Typically this is done through cross-validation.
/edit: For example, using cv.glment:
library(glmnet)
x <- model.matrix(Sepal.Length~., iris)[,-1]
y <- iris$Sepal.Length
mod <- cv.glmnet(as.matrix(x), y, alpha=1)

To see the coefficients with the minimum cross-validation error:
as.matrix(coef(mod, mod$lambda.min))
                           1
(Intercept)        2.1670759
Sepal.Width        0.5032347
Petal.Length       0.8137398
Petal.Width       -0.3127065
Speciesversicolor -0.6763395
Speciesvirginica  -0.9595409

To see the coefficients with the "largest value of lambda such that error is within 1 standard error of the minimum:"
as.matrix(coef(mod, mod$lambda.1se))
                            1
(Intercept)        2.14705035
Sepal.Width        0.59950383
Petal.Length       0.57550203
Petal.Width       -0.23632776
Speciesversicolor  0.00000000
Speciesvirginica  -0.04770282

You can also select any other value of lambda that you want.  Coefficients that are 0 have been dropped out of the model.  e.g.:
CF <- as.matrix(coef(mod, mod$lambda.1se))
CF[CF!=0,]
 (Intercept)      Sepal.Width     Petal.Length      Petal.Width Speciesvirginica 
  2.14705035       0.59950383       0.57550203      -0.23632776      -0.04770282 

If we uses the 1se lambda, the Speciesversicolor dummy variable gets dropped from the model.
